IBM 1620
From Wikipedia, the free encyclopedia
IBM 1620 Model I, Level H
IBM 1620 data processing machine with
IBM 1627 plotter, on display at the Seattle World's Fair, 1962
The
IBM 1620 was announced by
IBM
on October 21, 1959, and marketed as an inexpensive "scientific
computer". After a total production of about two thousand machines, it
was withdrawn on November 19, 1970. Modified versions of the 1620 were
used as the CPU of the
IBM 1710 and
IBM 1720 Industrial Process Control Systems (making it the first digital computer considered reliable enough for
real-time process control of factory equipment).
Being variable
word
length decimal, as opposed to fixed-word-length pure binary, made it an
especially attractive first computer to learn on — and hundreds of
thousands of students had their first experiences with a computer on the
IBM 1620.
[1]
Core memory cycle times were 20 microseconds for the
Model I, 10 microseconds for the
Model II (about a thousand times slower than typical computer main memory in 2006).
Many in the user community recall the 1620 being referred to as
CADET, jokingly meaning "
Can't
Add,
Doesn't
Even
Try", referring to the use of addition tables in memory rather than dedicated addition circuitry.
[2] For an explanation of all three known interpretations of the machine's code name see the section on the machine's
development history.
The 1620s architecture
It was a variable "
word" length decimal (
BCD)
computer with a memory that could hold anything from 20,000 to 60,000
decimal digits increasing in 20,000 decimal digit increments. (While the
5-digit addresses could have addressed 100,000 decimal digits, no
machine larger than 60,000 decimal digits was ever built.)
Memory was accessed two decimal digits at the same time (even-odd digit pair for numeric data or one
alphameric character for text data). Each decimal digit was 6 bits, composed of an odd parity
Check bit, a
Flag bit, and four BCD bits for the value of the digit in the following format:
C F 8 4 2 1
The
Flag bit had several uses:
- In the least significant digit it was set to indicate a negative number (signed magnitude).
- It was set to mark the most significant digit of a number (wordmark).
- In the least significant digit of 5-digit addresses it was set for indirect addressing (an option on the 1620 I,
standard on the 1620 II). Multi-level indirection could be used (you
could even put the machine in an infinite indirect addressing loop).
- In the middle 3 digits of 5-digit addresses (on the 1620 II) they were set to select one of 7 index registers.
In addition to the valid BCD digit values there were three
special digit values (these could NOT be used in calculations):
C F 8 4 2 1
1 0 1 0 - Record Mark (right most end of record)
1 1 0 0 - Numeric Blank (blank for punched card output formatting)
1 1 1 1 - Group Mark (right most end of a group of records for disk I/O)
Instructions were fixed length (12 decimal digits), consisting of a 2-digit "
op code", a 5-digit "P Address" (usually the
destination address), and a 5-digit "Q Address" (usually the
source address or the
source
immediate value). Some instructions, such as the B (branch)
instruction, only used the P Address, and later smart assemblers
included a "B7" instruction that generated an 7-digit branch instruction
(op code, P address, and one extra digit because the next instruction
had to start on an even-numbered digit).
Fixed-point data "words" could be any size from two decimal digits up to all of memory not used for other purposes.
Floating-point data "words" (using the hardware
floating point option) could be any size from 4 decimal digits up to 102 decimal digits (2 to 100 digits for the
mantissa and 2 digits for the
exponent).
The machine had no programmer-accessible registers: all operations were memory to memory (including the
index registers of the
1620 II).
Character and Op codes
The table below lists Alphameric mode Characters (and Op codes).
| BCD Character |
Typewriter |
Printer |
Tape |
Card |
Core |
MNEMONIC & Operation |
Definition & Notes |
| In |
Out |
Out |
In |
Out |
In |
Out |
Even |
Odd |
| Blank |
|
|
|
C |
C |
|
|
|
|
|
|
| invalid |
|
Ж ❚ |
|
|
1 |
FADD Floating Add |
Optional special feature. |
| invalid |
|
Ж ❚ |
|
|
2 |
FSUB Floating Subtract |
Optional special feature. |
| . |
. |
. |
. |
X0 8 21 |
X0 8 21 |
12-3-8
12-1-2-8 |
12-3-8 |
|
21 |
FMUL Floating Multiply |
Optional special feature. |
| ) |
) |
) |
) |
X0C84 |
X0C84 |
12-4-8 |
12-4-8 |
|
4 |
|
|
| invalid |
|
Ж ❚ |
|
|
4 1 |
FSL Floating Shift Left |
Optional special feature. |
| invalid |
|
Ж ❚ |
|
|
42 |
TFL Transmit Floating |
Optional special feature. |
| invalid |
|
Ж ❚ |
|
|
421 |
BTFL Branch and Transmit Floating |
Optional special feature. |
| invalid |
|
Ж ❚ |
|
|
8 |
FSR Floating Shift Right |
Optional special feature. |
| invalid |
|
Ж ❚ |
|
|
8 1 |
FDIV Floating Divide |
Optional special feature. |
| + |
+ |
+ |
+ |
X0C |
X0C |
12 |
12 |
1 |
|
|
|
| invalid |
|
Ж ❚ |
|
1 |
1 |
AM Add Immediate |
|
| invalid |
|
Ж ❚ |
|
1 |
2 |
SM Subtract Immediate |
|
| $ |
$ |
$ |
$ |
X C8 21 |
X C8 21 |
11-3-8
11-1-2-8 |
11-3-8 |
1 |
21 |
MM Multiply Immediate |
|
| * |
* |
* |
* |
X 84 |
X 84 |
11-4-8 |
11-4-8 |
1 |
4 |
CM Compare Immediate |
|
| invalid |
|
Ж ❚ |
|
1 |
4 1 |
TDM Transmit Digit Immediate |
|
| invalid |
|
Ж ❚ |
|
1 |
42 |
TFM Transmit Field Immediate |
|
| invalid |
|
Ж ❚ |
|
1 |
421 |
BTM Branch and Transmit Immediate |
|
| invalid |
|
Ж ❚ |
|
1 |
8 |
LDM Load Dividend Immediate |
Optional special feature (Model I).
Standard (Model II). |
| invalid |
|
Ж ❚ |
|
1 |
8 1 |
DM Divide Immediate |
Optional special feature (Model I).
Standard (Model II). |
| - |
- |
- |
- |
X |
X |
11 |
11 |
2 |
|
|
|
| / |
/ |
/ |
/ |
0C 1 |
0C 1 |
0-1 |
0-1 |
2 |
1 |
A Add |
|
| invalid |
|
Ж ❚ |
|
2 |
2 |
S Subtract |
|
| , |
, |
, |
, |
0C8 21 |
0C8 21 |
0-3-8
0-1-2-8 |
0-3-8 |
2 |
21 |
M Multiply |
|
| ( |
( |
( |
( |
0 84 |
0 84 |
0-4-8 |
0-4-8 |
2 |
4 |
C Compare |
|
| invalid |
|
Ж ❚ |
|
2 |
4 1 |
TD Transmit Digit |
|
| invalid |
|
Ж ❚ |
|
2 |
42 |
TF Transmit Field |
|
| invalid |
|
Ж ❚ |
|
2 |
421 |
BT Branch and Transmit |
|
| invalid |
|
Ж ❚ |
|
2 |
8 |
LD Load Dividend |
Optional special feature (Model I).
Standard (Model II). |
| invalid |
|
Ж ❚ |
|
2 |
8 1 |
D Divide |
Optional special feature (Model I).
Standard (Model II). |
| invalid |
|
Ж ❚ |
|
21 |
|
TRNM Transmit Record No RM |
(Model II) |
| invalid |
|
Ж ❚ |
|
21 |
1 |
TR Transmit Record |
|
| invalid |
|
Ж ❚ |
|
21 |
2 |
SF Set Flag |
|
| = |
= |
= |
= |
8 21 |
8 21 |
3-8
1-2-8 |
3-8 |
21 |
21 |
CF Clear Flag |
|
| @ |
@ |
@ |
@ |
C84 |
C84 |
4-8 |
4-8 |
21 |
4 |
K Control (I/O device) |
|
| invalid |
|
Ж ❚ |
|
21 |
4 1 |
DN Dump Numeric |
|
| invalid |
|
Ж ❚ |
|
21 |
42 |
RN Read Numeric |
|
| invalid |
|
Ж ❚ |
|
21 |
421 |
RA Read Alphameric |
|
| invalid |
|
Ж ❚ |
|
21 |
8 |
WN Write Numeric |
|
| invalid |
|
Ж ❚ |
|
21 |
8 1 |
WA Write Alphameric |
|
| A |
A |
A |
A |
X0 1 |
X0 1 |
12-1 |
12-1 |
4 |
1 |
NOP No Operation |
|
| B |
B |
B |
B |
X0 2 |
X0 2 |
12-2 |
12-2 |
4 |
2 |
BB Branch Back |
|
| C |
C |
C |
C |
X0C 21 |
X0C 21 |
12-3
12-1-2 |
12-3 |
4 |
21 |
BD Branch On Digit |
|
| D |
D |
D |
D |
X0 4 |
X0 4 |
12-4 |
12-4 |
4 |
4 |
BNF Branch No Flag |
|
| E |
E |
E |
E |
X0C 4 1 |
X0C 4 1 |
12-5
12-1-4 |
12-5 |
4 |
4 1 |
BNR Branch No Record Mark |
|
| F |
F |
F |
F |
X0C 42 |
X0C 42 |
12-6
12-2-4 |
12-6 |
4 |
42 |
BI Branch Indicator |
|
UMK Unmask
MK Mask |
1710 interrupt feature. Modifiers in Q field. |
| G |
G |
G |
G |
X0 421 |
X0 421 |
12-7
12-1-2-4 |
12-7 |
4 |
421 |
BNI Branch No Indicator |
|
BO Branch Out
BOLD Branch Out and Load |
1710 interrupt feature. Modifiers in Q field. |
| H |
H |
H |
H |
X0 8 |
X0 8 |
12-8 |
12-8 |
4 |
8 |
H Halt |
|
| I |
I |
I |
I |
X0C8 1 |
X0C8 1 |
12-9
12-1-8 |
12-9 |
4 |
8 1 |
B Branch |
|
| -0 |
N/A |
- |
- |
N/A |
X |
11-0 |
11-0 |
4 1 |
|
|
|
J
-1 |
J |
J |
J |
X C 1 |
X C 1 |
11-1 |
11-1 |
4 1 |
1 |
|
|
K
-2 |
K |
K |
K |
X C 2 |
X C 2 |
11-2 |
11-2 |
4 1 |
2 |
|
|
L
-3 |
L |
L |
L |
X 21 |
X 21 |
11-3
11-1-2 |
11-3 |
4 1 |
21 |
|
|
M
-4 |
M |
M |
M |
X C 4 |
X C 4 |
11-4 |
11-4 |
4 1 |
4 |
|
|
N
-5 |
N |
N |
N |
X 4 1 |
X 4 1 |
11-5
11-1-4 |
11-5 |
4 1 |
4 1 |
BNG Branch No Group Mark |
Optional special feature. |
O
-6 |
O |
O |
O |
X 42 |
X 42 |
11-6
11-2-4 |
11-6 |
4 1 |
42 |
|
|
P
-7 |
P |
P |
P |
X C 421 |
X C 421 |
11-7
11-1-2-4 |
11-7 |
4 1 |
421 |
|
|
Q
-8 |
Q |
Q |
Q |
X C8 |
X C8 |
11-8 |
11-8 |
4 1 |
8 |
|
|
R
-9 |
R |
R |
R |
X 8 1 |
X 8 1 |
11-9
11-1-8 |
11-9 |
4 1 |
8 1 |
|
|
| invalid |
|
Ж ❚ |
|
42 |
|
BS Branch and Select |
(Model II) |
| invalid |
|
Ж ❚ |
|
42 |
1 |
BX Branch and Modify Index Register |
Optional special feature (Model II). |
| S |
S |
S |
S |
0C 2 |
0C 2 |
0-2 |
0-2 |
42 |
2 |
BXM Branch and Modify Index Register Immediate |
Optional special feature (Model II). |
| T |
T |
T |
T |
0 21 |
0 21 |
0-3
0-1-2 |
0-3 |
42 |
21 |
BCX Branch Conditionally and Modify Index Register |
Optional special feature (Model II). |
| U |
U |
U |
U |
0C 4 |
0C 4 |
0-4 |
0-4 |
42 |
4 |
BCXM Branch Conditionally and Modify Index Register Immediate |
Optional special feature (Model II). |
| V |
V |
V |
V |
0 4 1 |
0 4 1 |
0-5
0-1-4 |
0-5 |
42 |
4 1 |
BLX Branch and Load Index Register |
Optional special feature (Model II). |
| W |
W |
W |
W |
0 42 |
0 42 |
0-6
0-2-4 |
0-6 |
42 |
42 |
BLXM Branch and Load Index Register Immediate |
Optional special feature (Model II). |
| X |
X |
X |
X |
0C 421 |
0C 421 |
0-7
0-1-2-4 |
0-7 |
42 |
421 |
BSX Branch and Store Index Register |
Optional special feature (Model II). |
| Y |
Y |
Y |
Y |
0C8 |
0C8 |
0-8 |
0-8 |
42 |
8 |
|
|
| Z |
Z |
Z |
Z |
0 8 1 |
0 8 1 |
0-9
0-1-8 |
0-9 |
42 |
8 1 |
|
|
| 0 |
0 |
0 |
0 |
0 |
0 |
0
12-0 |
0 |
421 |
|
MA Move Address |
Optional special feature (Model II). |
| 1 |
1 |
1 |
1 |
1 |
1 |
1 |
1 |
421 |
1 |
MF Move Flag |
Optional special feature (Model I).
Standard (Model II). |
| 2 |
2 |
2 |
2 |
2 |
2 |
2 |
2 |
421 |
2 |
TNS Transmit Numeric Strip |
Optional special feature (Model I).
Standard (Model II). |
| 3 |
3 |
3 |
3 |
C 21 |
C 21 |
3 |
3 |
421 |
21 |
TNF Transmit Numeric Fill |
Optional special feature (Model I).
Standard (Model II). |
| 4 |
4 |
4 |
4 |
4 |
4 |
4 |
4 |
421 |
4 |
|
|
| 5 |
5 |
5 |
5 |
C 4 1 |
C 4 1 |
5 |
5 |
421 |
4 1 |
|
|
| 6 |
6 |
6 |
6 |
C 42 |
C 42 |
6 |
6 |
421 |
42 |
|
|
| 7 |
7 |
7 |
7 |
421 |
421 |
7 |
7 |
421 |
421 |
|
|
| 8 |
8 |
8 |
8 |
8 |
8 |
8 |
8 |
421 |
8 |
|
|
| 9 |
9 |
9 |
9 |
C8 1 |
C8 1 |
9 |
9 |
421 |
8 1 |
|
|
| invalid |
|
Ж ❚ |
|
8 |
4 |
SA Select Address
SACO Select Address, Contact Operate
SAOS Select Analog Output Signal |
1710 feature. Modifiers in Q field |
| invalid |
|
Ж ❚ |
|
8 |
42 |
SLTA Select TAS
SLAR Select ADC Register
SLTC Select Real-Time Clock
SLIC Select Input Channel
SLCB Select Contact Block
SLME Select Manual Entry |
1710 feature. Modifiers in Q field |
| invalid |
|
Ж ❚ |
|
8 |
F 42 |
RNIC Read Numeric Input Channel |
1710 feature. Modifiers in Q field |
| invalid |
|
Ж ❚ |
|
8 |
F 421 |
RAIC Read Alphameric Input Channel |
1710 feature. Modifiers in Q field |
| invalid |
|
Ж ❚ |
|
8 |
8 |
WNOC Write Numeric Output Channel |
1710 feature. Modifiers in Q field |
| invalid |
|
Ж ❚ |
|
8 |
8 1 |
WAOC Write Alphameric Output Channel |
1710 feature. Modifiers in Q field |
| invalid |
|
Ж ❚ |
|
8 1 |
|
BBT Branch on Bit |
Optional special feature (Model II). |
| invalid |
|
Ж ❚ |
|
8 1 |
1 |
BMK Branch on Mask |
Optional special feature (Model II). |
| invalid |
|
Ж ❚ |
|
8 1 |
2 |
ORF OR to Field |
Optional special feature (Model II). |
| invalid |
|
Ж ❚ |
|
8 1 |
21 |
ANDF AND to Field |
Optional special feature (Model II). |
| invalid |
|
Ж ❚ |
|
8 1 |
4 |
CPLF Complement Octal Field |
Optional special feature (Model II). |
| invalid |
|
Ж ❚ |
|
8 1 |
4 1 |
EORF Exclusive OR to Field |
Optional special feature (Model II). |
| invalid |
|
Ж ❚ |
|
8 1 |
42 |
OTD Octal to Decimal Conversion |
Optional special feature (Model II). |
| invalid |
|
Ж ❚ |
|
8 1 |
421 |
DTO Decimal to Octal Conversion |
Optional special feature (Model II). |
| RM |
‡ |
(Stop) |
(Stop) |
0 8 2 |
E
(Stop) |
0-2-8 |
0-2-8 |
|
8 2 |
|
Record Mark |
| GM |
 |
(Stop) |
(Stop) |
0 8421 |
E
(Stop) |
0-7-8 |
1-2-4-8 |
|
8421 |
|
Group Mark |
The table below lists Numeric mode Characters.
| Character |
Typewriter |
Printer |
Tape |
Card |
Core |
Definition & Notes |
| In |
Out |
Out |
Dump |
In |
Out |
In |
Out |
| Blank |
|
0 |
0 |
0 |
C |
0 |
|
0 |
C |
|
| 0 |
0 |
0 |
0 |
0 |
0 |
0 |
0
12-0
12 |
0 |
C |
|
| 1 |
1 |
1 |
1 |
1 |
1 |
1 |
1
12-1 |
1 |
1 |
|
| 2 |
2 |
2 |
2 |
2 |
2 |
2 |
2
12-1 |
2 |
2 |
|
| 3 |
3 |
3 |
3 |
3 |
C 21 |
C 21 |
3
12-3
1-2
12-1-2 |
3 |
C 21 |
|
| 4 |
4 |
4 |
4 |
4 |
4 |
4 |
4
12-4 |
4 |
4 |
|
| 5 |
5 |
5 |
5 |
5 |
C 4 1 |
C 4 1 |
5
12-5
1-4
12-1-4 |
5 |
C 4 1 |
|
| 6 |
6 |
6 |
6 |
6 |
C 42 |
C 42 |
6
12-6
2-4
12-2-4 |
6 |
C 42 |
|
| 7 |
7 |
7 |
7 |
7 |
421 |
421 |
7
12-7
1-2-4
12-1-2-4 |
7 |
421 |
|
| 8 |
8 |
8 |
8 |
8 |
8 |
8 |
8
12-8 |
8 |
8 |
|
| 9 |
9 |
9 |
9 |
9 |
C8 1 |
C8 1 |
9
12-9
1-8
12-1-8 |
9 |
C 8 1 |
|
| -0 |
0 |
0 |
- |
- |
X
X0C |
X |
11-0 |
11-0 |
F |
|
| -1 |
1 |
1 |
J |
J |
X C 1 |
X C 1 |
11-1 |
11-1 |
CF 1 |
|
| -2 |
2 |
2 |
K |
K |
X C 2 |
X C 2 |
11-2 |
11-2 |
CF 2 |
|
| -3 |
3 |
3 |
L |
L |
X 21 |
X 21 |
11-3
11-1-2 |
11-3 |
F 21 |
|
| -4 |
4 |
4 |
M |
M |
X C 4 |
X C 4 |
11-4 |
11-4 |
CF 4 |
|
| -5 |
5 |
5 |
N |
N |
X 4 1 |
X 4 1 |
11-5
11-1-4 |
11-5 |
F 4 1 |
|
| -6 |
6 |
6 |
O |
O |
X 42 |
X 42 |
11-6
11-2-4 |
11-6 |
F 42 |
|
| -7 |
7 |
7 |
P |
P |
X C 421 |
X C 421 |
11-7
11-1-2-4 |
11-7 |
CF 421 |
|
| -8 |
8 |
8 |
Q |
Q |
X C8 |
X C8 |
11-8 |
11-8 |
CF8 |
|
| -9 |
9 |
9 |
R |
R |
X 8 1 |
X 8 1 |
11-9
11-1-8 |
11-9 |
F8 1 |
|
| RM |
‡ |
(Stop, WN)
‡
(DN) |
(Stop) |
‡ |
0 8 2 |
E
(Stop, WN)
0 8 2
(DN) |
0-2-8 |
0-2-8 |
C 8 2 |
Record Mark
On tape a WN punches EOL instead! |
| flag RM |
‡ |
(Stop, WN)
‡
(DN) |
(Stop) |
W |
X 8 2 |
E
(Stop, WN)
X 8 2
(DN) |
11-2-8
12-2-8 |
11-2-8 |
F8 2 |
Flagged Record Mark
On tape a WN punches EOL instead! |
| EOL |
‡ |
(Stop, WN)
‡
(DN) |
(Stop) |
‡ |
E |
E
(WN)
0 8 2
(DN) |
0-2-8 |
0-2-8 |
C 8 2 |
End of line Tape only.
Note: In memory is a Record Mark! |
| GM |
|
(Stop, WN)
(DN) |
(Stop) |
G |
0 8421 |
0 8421 |
0-7-8 |
0-7-8 |
C 8421 |
Group Mark |
| flag GM |
|
(Stop, WN)
(DN) |
(Stop) |
X |
X 8421 |
X 8421 |
12-7-8 |
12-7-8 |
F8421 |
Flagged Group Mark |
| NB |
@ |
@ |
|
@ |
C84 |
C84 |
4-8 |
|
C 84 |
Numeric Blank |
| flag NB |
@ |
@ |
|
* |
X 84 |
X 84 |
11-4-8 |
|
F84 |
Flagged Numeric Blank |
Invalid character
The
Model I used the
Cyrillic character Ж
(pronounced zh) on the typewriter as a general purpose invalid
character with correct parity (invalid parity being indicated with an
overstrike "–"). In some 1620 installations it was called a
SMERSH, as used in the
James Bond novels that had become popular in the late 1960s. The
Model II used a new character ❚ (called "pillow") as a general purpose invalid character with correct parity.
Architectural difficulties
Although the IBM 1620's architecture was very popular in the scientific and engineering community, computer scientist
Edsger Dijkstra pointed out several flaws in its design in EWD37, "A review of the IBM 1620 data processing system".
[3] Among these are that the machine's Branch and Transmit instruction together with Branch Back allow a grand total of
one
level of nested subroutine call, forcing the programmer of any code
with more than one level to decide where the use of this "feature" would
be most effective. He also showed how the machine's paper tape reading
support could not properly read tapes containing record marks, since
record marks are used to terminate the characters read in storage. (One
effect of this is that the 1620 cannot duplicate a tape with record
marks in a straightforward way: when the record mark is encountered, the
punch instruction punches an EOL character instead and terminates.
However this is not a crippling problem, as the data can be copied to
the end of memory and punched verbatim with a DN instruction instead of
WN. Besides, tapes were usually duplicated
offline.) Most 1620 installations used the more convenient punched card input/output, when it became available, rather than paper tape.
The successor to the 1620, the
IBM 1130 was based on a totally different, 16-bit binary architecture.
Software
IBM supplied the following software for the 1620:
- 1620 Symbolic Programming System (SPS) (assembly language)
- FORTRAN
- FORTRAN II - required 40,000 digits or more of memory
- GOTRAN - simplified, interpreted version of FORTRAN for "load and go" operation [4]
- Monitor I and Monitor II - disk operating systems.
The Monitors provided disk based versions of 1620 SPS IId, FORTRAN
IId as well as a DUP (Disk Utility Program). Both Monitor systems
required 20,000 digits or more of memory and 1 or more 1311 disk drives.
A collection of IBM 1620 related manuals in PDF format exists at
[5]
Operating procedures
The "operating system" for the computer constituted the operator, who would use controls on the computer
console, which consisted of a
front panel
and typewriter, to load programs from the available bulk storage media
such as decks of punched cards or rolls of paper tape that were kept in
cabinets nearby. Later, the model 1311 disc storage device attached to
the computer enabled a reduction in the fetch and carry of card decks or
paper tape rolls, and a simple "Monitor" operating system could be
loaded to help in selecting what to load from disc.
A standard preliminary was to clear the computer memory of any
previous user's detritus - being magnetic cores, the memory retained its
last state even if the power had been switched off. This was effected
by using the console facilities to load a simple computer program via
typing its machine code at the console typewriter, running it, and
stopping it. This was not challenging as only one instruction was needed
such as 160001000000, loaded at address zero and following. This meant
transmit field immediate
(the 16: two digit op-codes) to address 00010 the immediate constant
field having the value 00000 (five digit operand fields, the second
being from address 11 back to 7), decrementing source and destination
addresses until such time as a digit with a "flag" was copied. This was
the normal machine code means of copying a constant of up to five
digits. The digit string was addressed at its low-order end and extended
through lower addresses until a digit with a flag marked its end. But
for this instruction, no flag would ever be found because the source
digits had shortly before been overwritten by digits lacking a flag.
Thus the operation would roll around memory (even overwriting itself)
filling it with all zeroes until the operator grew tired of watching the
roiling of the indicator lights and pressed the
Instant Stop - Single Cycle Execute button. Each 20,000 digit module of memory took just under one second to clear. On the
1620 II this instruction would
NOT work (due to certain optimizations in the implementation). Instead there was a button on the console called
Modify which when pressed together with the
Check Reset
button, when the computer was in Manual mode, would set the computer in
a mode that would clear all of memory in a tenth of one second
regardless of how much memory you had; when you pressed
Start. It also stopped automatically when memory was cleared, instead of requiring the operator to stop it.
Other than typing machine code at the console, a program could be
loaded via either the paper tape reader, the card reader, or any disk
drive. Loading from either tape or disk required first typing a "
bootstrap" routine on the console typewriter.
The card reader made things easier because it had a special
Load
button to signify that the first card was to be read into the
computer's memory (starting at address 00000) and executed (as opposed
to just starting the card reader, which then awaits commands from the
computer to read cards) - this is the "bootstrap" process that gets into
the computer just enough code to read in the rest of the code (from the
card reader, or disc, or...) that constitutes the loader that will read
in and execute the desired program.
Programs were prepared ahead of time, offline, on paper tape or
punched cards. But usually the programmers were allowed to run the
programs personally, hands-on, instead of submitting them to operators
as was the case with mainframe computers at that time. And the console
typewriter allowed entering data and getting output in an interactive
fashion, instead of just getting the normal printed output from a blind
batch run on a pre-packaged data set. As well, there were four
program switches
on the console whose state a running program could test and so have its
behavior directed by its user. The computer operator could also stop a
running program (or it may come to a deliberately programmed stop) then
investigate or modify the contents of memory: being decimal-based, this
was quite easy; even floating-point numbers could be read at a glance.
Execution could then be resumed, from any desired point. Aside from
debugging, scientific programming is typically exploratory, by contrast
to commercial data processing where the same work is repeated on a
regular schedule.
Console
IBM 1620 Memory address register display selector switch.
- For details of console lights, switches, and procedures see the respective articles on the IBM 1620 Model I or IBM 1620 Model II.
The most important items on the 1620's console were a pair of buttons labeled
Insert &
Release, and the electric typewriter.
- Insert – Pressing this key with the computer in Manual mode reset the program counter (in the MARS core memory) to zero, switched the computer into Automatic and Insert
modes, and simulated the execution of a Read Numeric from Typewriter to
address zero (unlocked the typewriter keyboard, shifted the typewriter
into numeric mode). Note: unlike a real Read Numeric from Typewriter, Insert mode would force a Release after 100 digits had been typed to prevent overwriting the arithmetic tables.
- Release – Pressing this key while doing a Read from the Typewriter terminated the Read, switched the computer into Manual mode, and locked the typewriter keyboard.
The typewriter is used for operator input/output, both as the main
console control of the computer and for program controlled input/output.
Later models of the typewriter had a special key marked
R-S that combined the functions of the console
Release &
Start buttons (this would be considered equivalent to an
Enter key on a modern keyboard). Note: several keys on the typewriter did not generate input characters, these included
Tab and
Return (the 1620's alphameric and numeric BCD character sets lacked character codes for these keys).
The next most important items on the 1620's console were the buttons labeled
Start,
Stop-SIE, and
Instant Stop-SCE.
- Start – Pressing this key with the computer in Manual mode switched the computer to Automatic mode (causing the computer to begin executing at the address in the program counter).
- Stop-SIE – Pressing this key with the computer in Automatic mode switched the computer to Manual mode when the currently executing instruction completes. Pressing this key with the computer in Manual mode switched the computer into Automatic mode for one instruction.
- Instant Stop-SCE – Pressing this key with the computer in Automatic mode switched the computer into Automatic/Manual mode at the end of the current memory cycle. Pressing this key with the computer in Manual or Automatic/Manual mode switched the computer into Automatic/Manual mode and executed one memory cycle.
For program debugging there were the buttons labeled
Save &
Display MAR.
- Save – Pressing this key with the computer in Manual mode saved the program counter into another register in the MARS core memory and activated Save mode.
When a Branch Back instruction was executed in
Save mode, it
copied the saved value back to the program counter (instead of copying
the return address register as it normally did) and deactivated
Save mode.
This was used during debugging to remember where the program had been
stopped to allow it to be resumed after the debugging instructions that
the operator had typed on the typewriter had finished. Note: the MARS
register used to save the program counter in was also used by the
Multiply instruction, so this instruction and the
Save
mode were incompatible! However there was no need to use multiply in
debugging code, so this was not considered to be a problem.
- Display MAR – Pressing this key with the computer in Manual mode displayed the selected MARS register and the contents of the memory at that address on the console lamps.
| "Breakpoint" procedure |
Notes |
| Press Stop-SIE |
Stop the computer at the end of the current instruction. |
| Press Save |
Save the address to resume execution at. |
| Press Insert |
Unlocks typewriter keyboard and shifts into numeric mode. |
Type 35xxxxx0010036xxxxx0010042
|
xxxxx is the address that you plan to set the breakpoint at. |
| Press Release |
Locks typewriter keyboard. |
| Press Start |
Begin execution. Allow the 12 digit instruction to print out. |
| Press Release |
Stops the Dump Numeric. |
| Press Start |
Begin execution. |
| Type 48 |
Replace the opcode of the instruction to "break" at with a Halt opcode. |
| Press Release |
Locks typewriter keyboard. |
| Press Start |
Resume execution. Wait until the computer halts at the "breakpoint". |
| Press Insert |
Unlocks typewriter keyboard and shifts into numeric mode. |
Type 36xxxxx0010049xxxxx
|
xxxxx is the address that you previously set the breakpoint at, you are now going to clear it. |
| Press Release |
Locks typewriter keyboard. |
| Press Start |
Begin execution. |
| Type oo |
oo is the 2 digit opcode the original 12 digit instruction previously printed out. |
| Press Release |
Locks typewriter keyboard. |
| Press Stop-SIE |
The machine is now ready to resume execution from the location of
the (now cleared) "breakpoint". You can perform any required debugging
actions now, before continuing. |
Paper Tape reader/punch
The
1621 Tape reader and
1624 Tape punch controls.
- Power switch – With this switch on the reader is powered anytime the 1620 is powered.
- Reel-Strip switch – This switch selects whether Reels or Strips of paper tape are used.
- Reel power key – Applies power to the supply and takeup Reels to
position the tape for reading and places the reader in ready state.
- Non-process runout key – Feeds tape until the reader is empty and takes the reader out of ready state.
| Bootstrap procedure |
Notes |
| Press Insert |
Unlocks typewriter keyboard and shifts into numeric mode. |
Type 36xxxxx0030049yyyyy
|
xxxxx is the address to load the tape into.
yyyyy is the address to begin execution. |
| Press Release |
Locks typewriter keyboard. |
| Press Start |
Begin execution. |
Card reader/punch
The
1622
Card reader/punch controls were divided into three groups: 3 punch
control rocker switches, 6 buttons, and 2 reader control rocker
switches.
Punch Rocker switches:
- Punch Off/Punch On – This rocker turned the punch mechanism off or on.
- Select No-Stop/Select Stop – This rocker selected if mispunched
cards (deposited in the punch error select stacker instead of the normal
punch stacker) let the punch continue or caused a check stop.
- Non-Process Runout – This rocker with the punch hopper empty, "ranout" remaining cards from the punch mechanism.
Buttons:
- Start punch – Pressing this key with the punch idle and on, started the punch. The computer could now punch cards.
- Stop punch – Pressing this key with the punch active, stopped the punch.
- Check Reset – Pressing this key reset all "error check" conditions in the reader and punch.
- Load – Pressing this key with the reader idle and on and the computer in Manual
mode started the reader, reset the program counter (in the MARS core
memory) to zero, read one card into the reader's buffer and checked the
card for errors, and simulated the execution of a Read Numeric from Card
Reader to address zero (reading the 80 characters of the reader's
buffer into memory addresses 00000 to 00079), then switched the computer
into Automatic mode (starting execution at the address in the program counter).
- Stop reader – Pressing this key with the reader active, stopped the reader.
- Start reader – Pressing this key with the reader idle and on,
started the reader and read one card into the reader's buffer and
checked the card for errors. The computer could now read cards.
Reader Rocker switches:
- Non-Process Runout – This rocker with the read hopper empty, "ranout" remaining cards from the reader mechanism.
- Reader Off/Reader On – This rocker turned the reader mechanism off or on.
| Bootstrap procedure |
Notes |
| Press Load |
|
Disk drives
IBM 1311 Disk Drives - Model 2 (Slave) & Model 3 (Master), attached to an IBM 1620 II
The
1311 Disk drive controls.
- Module light – This light shows the drive number. When it lights the drive is ready for access.
- Compare-Disable key-switch – When this (Master only) switch is in
the ON position and the Write Address button is pressed a full track
write may be performed without comparing addresses. Used to format disk
packs.
- Select Lock light – When this (Master only) lights one or more of the drives has malfunctioned. No disk access can be performed.
- Write Address button/light – This (Master only) key controls writing
sector addresses. Pressing it toggles this enable and turns its light
on/off.
- Enable-Disable toggle-switch – This switch enables or disables
access to the drive. If this switch is disabled on the Master, all
drives are disabled regardless of the state of their own switches. Also
controls the disk usage time meter(s).
- Start Stop button – Pressing this key starts or stops the disk drive
motor. The motor must be stopped to open the lid and change disk packs.
| Bootstrap procedure |
Notes |
| Press Insert |
Unlocks typewriter keyboard and shifts into numeric mode. |
Type 3400032007013600032007024902402
x
y1963611300102 |
x - Specifies source of Monitor control cards: 1=typewriter, 3=paper tape, 5=cards
y - Specifies disk drive on which Monitor resides: 1, 3, 5, 7
02402 is the address of the entry point of the Monitor program. |
| Press Release |
Locks typewriter keyboard. |
| Press Start |
Begin execution. |
| Restart procedure |
Notes |
| Press Insert |
Unlocks typewriter keyboard and shifts into numeric mode. |
Type 490225FLG6
|
02256̅ is the address of the location containing the address of the restart point of the Monitor program.
Note: this procedure assumes the Monitor is already loaded in memory |
| Press Release |
Locks typewriter keyboard. |
| Press Start |
Begin execution. |
General
The FORTRAN II compiler and SPS assembler were somewhat cumbersome to use
[6][7]
by modern standards, however, with repetition, the procedure soon
became automatic and you no longer thought about the details involved.
| FORTRAN II compilation procedure |
Notes |
Set the Program Switches as follows:
- OFF (No Source listing)
- OFF (No Batch compilation)
- OFF (Source is entered from cards)
- OFF (Only used if 3 is ON)
|
Pass I options |
| Set Overflow Check switch to Program and all others to Stop |
|
| Press Reset |
|
| Load blank cards (face down 12-edge first) into the Punch hopper then press Punch Start |
|
| Load Pass I of the compiler (face down 9-edge first) into the Read hopper then press Load |
Wait for Pass I to load and print on the typewriter "ENTER SOURCE PROGRAM, PRESS START" |
| Remove Pass I of the compiler from the Read stacker |
|
| Load the program source deck (face down 9-edge first) into the Read hopper then press Start |
Wait for Pass I to complete and print on the typewriter "TURN SW 1 ON FOR SYMBOL TABLE, PRESS START" |
| Turn Program Switch 1 OFF then press Start |
If a symbol table listing is desired for debugging, turn Program Switch 1 ON instead. The symbol table listing will be printed on the typewriter.
Wait for Pass I to print on the typewriter "END OF PASS 1" |
Set the Program Switches as follows:
- OFF (No statement number/address listing)
- OFF (Not used)
- OFF (No trace for IF statements)
- OFF (No trace for arithmetic statements)
|
Pass II options |
| Set Overflow Check switch to Program and all others to Stop |
|
| Press Reset |
|
| Load blank cards (face down 12-edge first) into the Punch hopper then press Punch Start |
|
| Load Pass II of the compiler (face down 9-edge first) into the Read hopper then press Load |
Wait for Pass II to load |
| Remove Pass II of the compiler from the Read stacker |
|
| Remove the intermediate output of Pass I from the Punch stacker,
then load it (face down 9-edge first) into the Read hopper and press Reader Start then Start |
Wait for Pass II to complete and print on the typewriter "SW 1 ON TO PUNCH SUBROUTINES, PRESS START" |
| Remove the intermediate output from the Reader stacker |
|
| Turn Program Switch 1 ON, load the Subroutine deck (face down 9-edge first) into the Read hopper, then press Reader Start then Start |
Wait for Pass II to print on the typewriter "END OF PASS II" |
| Remove the Subroutine deck from the Reader stacker and the completed Object deck from the Punch stacker |
|
GOTRAN was much simpler to use, as it directly produced an executable
in memory. However it was not a complete FORTRAN implementation.
To improve this various third-party FORTRAN compilers were developed.
One of these was developed by Bob Richardson, a programmer at
Rice University,
the FLAG (FORTRAN Load-and-Go) compiler. Once the FLAG deck had been
loaded, all that was needed was to load the source deck to get directly
to the output deck; FLAG stayed in memory, so it was immediately ready
to accept the next source deck. This was particularly convenient for
dealing with many small jobs. For instance, at
Auckland University
a batch job processor for student assignments (typically, many small
programs not requiring much memory) chugged through a class lot rather
faster than the later
IBM 1130
did with its disk-based system. The compiler remained in memory, and
the student's program had its chance in the remaining memory to succeed
or fail, though a bad failure might disrupt the resident compiler.
Later, disk storage devices were introduced, removing the need for
working storage on card decks. The various decks of cards constituting
the compiler and loader no longer need be fetched from their cabinets
but could be stored on disk and loaded under the control of a simple
disk-based operating system: a lot of activity becomes less visible, but
still goes on.
Since the punch side of the card reader-punch didn't edge-print the
characters across the top of the cards, one had to take any output decks
over to a
separate machine, typically an
IBM 557
Alphabetic Interpreter, that read each card and printed its contents
along the top. Listings were usually generated by punching a listing
deck and using an
IBM 407 accounting machine to print the deck.
Hardware implementation
Most of the logic circuitry of the 1620 was a type of
resistor-transistor logic (RTL) using
"drift" transistors (a type of transistor invented by
Herbert Kroemer in 1953) for their speed, that IBM referred to as
SDTRL. Other IBM circuit types used were referred to as:
Alloy (some logic, but mostly various non-logic functions, named for the kind of transistors used),
CTRL (another type of RTL, but slower than
SDTRL),
CTDL (a type of
diode-transistor logic (DTL)), and
DL (another type of RTL, named for the kind of transistor used, "drift" transistors). Typical logic levels of all these circuits (
S Level) were high: 0 V to -0.5 V, low: -6 V to -12 V.
Transmission line logic levels of
SDTRL circuits (
C Level) were high: 1 V, low: -1 V. Relay circuits used either of two logic levels (
T Level) high: 51 V to 46 V, low: 16 V to 0 V or (
W Level) high: 24 V, low: 0 V.
These circuits were constructed of individual discrete components mounted on single sided paper-epoxy
printed circuit boards 2.5 by 4.5 inches (64 by 114 millimetres) with a 16-pin
gold plated
edge connector, that IBM referred to as
SMS cards (
Standard Modular System). The amount of logic on one card was similar to that in one
7400 series SSI or simpler MSI package (e.g., 3 to 5 logic gates or a couple of flip-flops).
These boards were inserted into sockets mounted in door-like racks which IBM referred to as
gates. The machine had the following "gates" in its basic configuration:
- "Gate A" - Forward hinged gate that swings out the back for access, after "Gate B".
- "Gate B" - Rear hinged gate that swings out the back for access.
- "Gate C" - Slides out back for access. Console Typewriter interface. Mostly relay logic.
- "Gate D" - Slides out back for access. Standard I/O interface.
There were two different types of
core memory used in the 1620:
- Main memory
- Coincident Current X-Y Line addressing
- 20,000, 40,000, or 60,000 Digits
- 12-bit, even-odd Digit Pair
- 12 one bit planes in each module, 1 to 3 modules
- Memory Address Register Storage (MARS) memory
- Word Line addressing
- 16 Words, minimum of 8 used in basic configuration
- Single Word read, multiple Word clear/write
- 24-bit, 5 Digit decimal Memory Address (no 8 - Ten Thousand bit stored)
- 1 plane
The address decoding logic of the Main memory also used two planes of 100
pulse transformer cores per module to generate the X-Y Line half-current pulses.
There were two models of the 1620, each having totally different hardware implementations:
Development history
A computer for the "small scientific market"
In 1958 IBM assembled a team at the
Poughkeepsie, New York
development laboratory to study the "small scientific market".
Initially the team consisted of Wayne Winger (Manager), Robert C.
Jackson, and William H. Rhodes.
Requirements and design
The competing computers in this market were the
Librascope LGP-30 and the
Bendix G-15; both were
drum memory machines. IBM's smallest computer at the time was the popular
IBM 650, a fixed word length decimal machine that also used drum memory. All three used
vacuum tubes.
It was concluded that IBM could offer nothing really new in that area.
To compete effectively would require use of technologies that IBM had
developed for larger computers, yet the machine would have to be
produced at the least possible cost.
To meet this objective, the team set the following requirements:
- Core memory
- Restricted instruction set
- No divide or floating point instructions, use subroutines in the "general program package"
- Wherever possible replace hardware with existing logical machine functions
- No arithmetic circuits, use tables in core memory
- Least expensive Input/Output possible
- No punch cards, use paper tape
- No printer, use operator's console typewriter
The internal code name
CADET was selected for the machine. One of the developers says that this stood for "
Computer with
ADvanced
Economic
Technology", however others recall it as simply being one half of
"SPACE - CADET", where
SPACE was the internal code name of the
IBM 1401 machine, also then under development.
The prototype
The team expanded with the addition of Anne Deckman, Kelly B. Day,
William Florac, and James Brenza. They completed the CADET prototype in
the spring of 1959.
Meanwhile the
San Jose, California facility was working on a proposal of its own. IBM could only build one of the two and the
Poughkeepsie
proposal won because "the San Jose version is top of the line and not
expandable, while your proposal has all kinds of expansion capability -
never offer a machine that cannot be expanded".
 |
IBM 1620 Model I Level A (prototype), as it appeared
in the IBM announcement of the machine.
|
Management was not entirely convinced that
core memory could be made to work in small machines, so Gerry Ottaway was loaned to the team to design a
drum memory as a backup. During
acceptance testing
by the Product Test Lab, repeated core memory failures were encountered
and it looked likely that management's predictions would come true.
However, at the last minute it was found that the
muffin fan
used to blow hot air through the core stack was malfunctioning, causing
the core to pick up noise pulses and fail to read correctly. After the
fan problem was fixed, there were no further problems with the core
memory and the drum memory design effort was discontinued as
unnecessary.
Transferred to San Jose for production
Following announcement of the IBM 1620 on October 21, 1959, due to an
internal reorganization of IBM, it was decided to transfer the computer
from the Data Processing Division at Poughkeepsie (large scale
mainframe computers only) to the General Products Division at San Jose
(small computers and support products only) for manufacturing.
Following transfer to San Jose, someone there jokingly suggested that the code name
CADET actually stood for "
Can't
Add,
Doesn't
Even
Try",
referring to the use of addition tables in memory rather than dedicated
addition circuitry. This stuck and became very well known among the
user community.
Implementation "levels"
- Model I
- Level A; prototype.
- All flip-flops in the design were transistorized versions of the original Eccles-Jordan trigger circuit. While this machine was fully functional, it was found that the capacitor coupling used in these proved troublesome in the noisy signal environment of relays and timing cam driven switches used to drive the console typewriter. This necessitated a complete redesign of the machine to use S-R flip-flops instead (except for two triggers used to generate clocks for the S-R flip-flops). However usage of the term Trigger was retained in all the documentation when referring to a flip-flop, as it was IBM's conventional term (as alphamerics was their term for alphanumerics).
- This is the only level using a one piece vertical control panel,
when the design was transferred from Poughkeepsie to San Jose it was
redesigned to the two piece angled control panel used on all production
models.
- Level B; first production.
- This is the only level using a burnished aluminum lower control panel, later levels finished this panel with white.
- Level C; introduction of 1622 card reader/punch.
- Level D; introduction of 1311 disk drives and addition of optional "Gate J" containing disk control logic.
- Level E; introduction of Floating Point option.
- Level F
- Level G; introduction of Interrupt option (needed for IBM 1710).
- Did not support BT & BB subroutines in interrupt code!
- Disk control logic on "Gate J" logic merged into "Gate A" & "Gate B".
- Made possible because much of logic was compacted using cards designed for the Model II.
- Level H; improved Interrupt option that supported BT & BB subroutines in interrupt code.
- Final version of the Model I.
- Model II (no information on "Levels" available at this time)
-
- The 1620 Model II introduced basic ALU hardware for addition and subtraction (making "Can't Add, Doesn't Even Try" no longer applicable) and index registers.
- Model III
- Work was begun on a 1620 Model III but the project was quickly canceled as IBM wanted to promote sales of their new System/360 and discontinue the old lines.
Patents
- U.S. Patent 3,049,295 - Multiplying Computer
- Patent filed: December 20, 1960
- Patent issued: August 14, 1962
- Inventors
- William H. Rhodes
- James G. Brenza
- Wayne D. Winger
- Robert C. Jackson
- Claims and prior art references
- Diagrams and Text
- 156 sheets of diagrams (Describes 1620 in full details.)
- 31 sheets of text
- U.S. Patent 3,328,767 - Compact Data Lookup Table
- Patent filed: December 31, 1963
- Patent issued: June 27, 1967
- Inventors
- Claims and prior art references
- Diagrams and Text
- 5 sheets of diagrams
- 4 sheets of text
|
- U.S. Patent 3,199,085 - Computer with Table Lookup Arithmetic Unit Feature
- Patent filed: December 20, 1960
- Patent issued: August 3, 1965
- Inventors
- William H. Rhodes
- James G. Brenza
- Wayne D. Winger
- Claims and prior art references
- Diagrams and Text
- 156 sheets of diagrams (Describes 1620 in full details.)
- 31 sheets of text
- U.S. Patent 3,239,654 - Dividing Computer
- Patent filed: February 8, 1961
- Patent issued: March 8, 1966
- Inventors
- Robert C. Jackson
- William A. Florac
- Wayne D. Winger
- Claims and prior art references
- 9 claims
- 1 prior art
- 3 publications
- Diagrams and Text
- 13 sheets of diagrams
- 19 sheets of text
|
Related peripheral units
Available peripherals were:
- IBM 1621 - Paper tape reader
- IBM 1622 - Punch card reader/punch
- IBM 1624 - Paper tape punch (sat inside the 1621 on a shelf)
- IBM 1626 - Plotter controller
- IBM 1627 - Plotter
- IBM 1311 - Disk drive: Model 3 master drive controlling up to 3–Model 2 slave drives.
- IBM 1443 - Printer, flying type bar
- IBM 1405 - Disk Drive available as RPQ (request price quotation)
Notable Uses
An IBM 1620 model II was used by Vearl N. Huff, NASA Headquarters
(FOB 10B, Washington DC) to program a three-dimensional simulation in
Fortran of the tethered Gemini capsule - Agena rocket module two-body
problem at a time when it was not completely understood if it was safe
to tether two objects together in space due to possible elastic tether
induced collisions. The same computer was also used to simulate the
orbits of the Gemini flights, producing printer-art charts of each
orbit. These simulation were run over-night and the data examined the
next day.
[8]
Use in film and television
A radio program was developed by DJ Rege Cordic for KDKA Pittsburgh,
based on a baseball game simulator developed by John Burgeson of IBM and
his brother, Paul, then an ensign in the U.S. Navy. This program was
used in numerous demonstration events in the years 1960 to 1963 as an
example of the power of computers to perform simulation exercises. The
fictional computer Colossus of
Colossus: The Forbin Project used about a dozen scrapped 1620 front panels purchased on the surplus market, in various orientations.
[9] A similar arrangement was used in a late episode
[citation needed] of
The Man from U.N.C.L.E. to portray a
THRUSH supercomputer.
References
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