IBM 1620 Model II
Encyclopedia
The IBM 1620 Model II was a vastly improved implementation, compared to the original Model I
, of the IBM 1620
scientific computer architecture.
It had basic ALU hardware for addition and subtraction, but multiplication was still done by table lookup in core memory. Multiplication used a 200 digit table (@ address 00100..00299). Rather than being an available option, as in the Model I, the divide hardware using a repeated subtraction algorithm, was built in. Floating point
arithmetic was an available option, as were octal input/output, logical operations, and base conversion to/from decimal instructions.
The core memory (@ address 00300..00399) that was freed by the replacement of the addition table with hardware was used for storage of two selectable "bands" of seven 5-digit index register
s.
The console typewriter was replaced with a modified Selectric
typewriter, which could type at 15.5 cps — a 55% improvement over the Model I.
The entire core memory was in the IBM 1625 memory unit. Memory cycle time was halved compared to the Model I's (internal or 1623 memory unit), to 10 µs
(i.e., the cycle speed was raised to 100 kHz) by using faster cores. A Memory Address Register Storage (MARS) core memory read, clear, or write operation took 1.5 µs and each write operation was automatically (but not necessarily immediately) preceded by a read or clear operation of the same "register(s)" during the 10 µs memory cycle.
The processor clock speed was also doubled, to 2 MHz, which was still divided by 20 by a 10 position ring counter
to provide the system timing/control signals.
The fetch/execute mechanism was completely redesigned, optimizing the timing and allowing partial fetches when the P or Q fields were not needed. Instructions took either 1, 4, or 6 Memory cycles (10 µs, 40 µs, or 60 µs) to fetch and a variable number of memory cycles to execute. Indirect addressing added 3 memory cycles (30 µs) for each level of indirection
. Indexed addressing added 5 memory cycles (50 µs) for each level of indexing. Indirect and indexed addressing could be combined at any level of indirection or indexing.
input/output, logical operations, and base conversion to/from decimal was available. This made the Model II very practical for applications that needed to manipulate data formatted in octal by other computers (e.g., the IBM 7090
).
Bases other than 8 and 10 were not supported.
IBM 1620 Model I
The IBM 1620 Model I was the original implementation of the IBM 1620 scientific computer, introduced in 1959.This unit was produced as inexpensively as IBM could make it, in order to keep the price low...
, of the IBM 1620
IBM 1620
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...
scientific computer architecture.
It had basic ALU hardware for addition and subtraction, but multiplication was still done by table lookup in core memory. Multiplication used a 200 digit table (@ address 00100..00299). Rather than being an available option, as in the Model I, the divide hardware using a repeated subtraction algorithm, was built in. Floating point
Floating point
In computing, floating point describes a method of representing real numbers in a way that can support a wide range of values. Numbers are, in general, represented approximately to a fixed number of significant digits and scaled using an exponent. The base for the scaling is normally 2, 10 or 16...
arithmetic was an available option, as were octal input/output, logical operations, and base conversion to/from decimal instructions.
The core memory (@ address 00300..00399) that was freed by the replacement of the addition table with hardware was used for storage of two selectable "bands" of seven 5-digit index register
Index register
An index registerCommonly known as a B-line in early British computers. in a computer's CPU is a processor register used for modifying operand addresses during the run of a program, typically for doing vector/array operations...
s.
The console typewriter was replaced with a modified Selectric
IBM Selectric typewriter
The IBM Selectric typewriter was a highly successful model line of electric typewriters introduced by IBM on July 31, 1961.Instead of the "basket" of individual typebars that swung up to strike the ribbon and page in a traditional typewriter, the Selectric had a type element that rotated and...
typewriter, which could type at 15.5 cps — a 55% improvement over the Model I.
The entire core memory was in the IBM 1625 memory unit. Memory cycle time was halved compared to the Model I's (internal or 1623 memory unit), to 10 µs
Microsecond
A microsecond is an SI unit of time equal to one millionth of a second. Its symbol is µs.A microsecond is equal to 1000 nanoseconds or 1/1000 millisecond...
(i.e., the cycle speed was raised to 100 kHz) by using faster cores. A Memory Address Register Storage (MARS) core memory read, clear, or write operation took 1.5 µs and each write operation was automatically (but not necessarily immediately) preceded by a read or clear operation of the same "register(s)" during the 10 µs memory cycle.
The processor clock speed was also doubled, to 2 MHz, which was still divided by 20 by a 10 position ring counter
Ring counter
A ring counter is a type of counter composed of a circular shift register. The output of the last shift register is fed to the input of the first register.There are two types of ring counters:...
to provide the system timing/control signals.
The fetch/execute mechanism was completely redesigned, optimizing the timing and allowing partial fetches when the P or Q fields were not needed. Instructions took either 1, 4, or 6 Memory cycles (10 µs, 40 µs, or 60 µs) to fetch and a variable number of memory cycles to execute. Indirect addressing added 3 memory cycles (30 µs) for each level of indirection
Indirection
In computer programming, indirection is the ability to reference something using a name, reference, or container instead of the value itself. The most common form of indirection is the act of manipulating a value through its memory address. For example, accessing a variable through the use of a...
. Indexed addressing added 5 memory cycles (50 µs) for each level of indexing. Indirect and indexed addressing could be combined at any level of indirection or indexing.
Non-decimal arithmetic
Unlike the Model I addition and subtraction were now fully implemented in hardware, so changing the table in memory could not be used as a "trick" to change arithmetic bases. However an optional special feature in hardware for octalOctal
The octal numeral system, or oct for short, is the base-8 number system, and uses the digits 0 to 7. Numerals can be made from binary numerals by grouping consecutive binary digits into groups of three...
input/output, logical operations, and base conversion to/from decimal was available. This made the Model II very practical for applications that needed to manipulate data formatted in octal by other computers (e.g., the IBM 7090
IBM 7090
The IBM 7090 was a second-generation transistorized version of the earlier IBM 709 vacuum tube mainframe computers and was designed for "large-scale scientific and technological applications". The 7090 was the third member of the IBM 700/7000 series scientific computers. The first 7090 installation...
).
Bases other than 8 and 10 were not supported.
Upper console
- Control Gates – 60 lamps
- Input-Output – 35 lamps
- Inst & Exec Cycle – 15 lamps
- Operation Register – 25 lamps
- Memory Buffer Register – 30 lamps
- Memory Address Register – 25 lamps
- Memory Address Register Display Selector – Rotary switch, 12 positions
Lower console
- Emergency Off Pull – Pull switch
- Check Condition status lamps/switches – 15 lamps & 5 toggle switches
- Program Switches – 4 toggle switches
- Console operator lights/switches – 13 lights, 1 power switch, and 12 buttons