Glenn’s Computer Museum

The IBM System/32: The Second IBM Personal Computer

Announced in early 1975, the IBM System/32 was the lowest cost IBM offering (at the time) and became very popular.

It was a small single-user business-oriented computer that was less expensive than the System/3 Model 6 (some models leased for less than $1,000 per month. The System/32 was designed for business applications (e.g., billing, inventory control, accounts receivable, sales analysis, etc.) written in RPG.

The system was a single desk-sized unit comprising a processor, fixed-disk storage, a data diskette, a printer, and an operator console with a small visual display screen. The System'32's tiny display contained six lines of 40 characters each. Solid-state main storage was available in sizes from 16K to 32K bytes. A emovable contained around 262 KB, and the fixed disk options were approximately 5 or 9MB. Two communication options were available (BSCA and SDLC) supporting data rates up to 7200 bps.

Figure 1 shows the complete System/32 (the weird photo angle is to mask a huge amount of junk in the room). Figure 2 shows the operator station with the small display on the left and the printer behind the keyboard, which is shown in more detail in Figure 3. Figure 5 shows the hidden CE panel (in front of which I spent many nights, see below). Swinging the logic gate out, we see in Figure 5 the giant disk drive. Figure 6 and 7 show the front and back of the logic gate. This small amount is the entire logic in the system (other than the CRT control logic and without the optional communications feature). Figure 8 shows the back of the unit with the vertically mounted CRT in a shield to the left of the disk drive. The motor for the diskette reader is alos seen below the CRT shield.

From an application viewpoint, the System/32 had the same instruction set as the System/3 models and the system software (SCP) was very similar to that of the System/3 Model 6's. Similarly, the primary application language was RPG. Due to its small business orientation and low cost, the System/32 was very successful: according to the IBM link, it became the most installed IBM computer of the 1970's.

Even though the System/32 appeared to software to have the System/3 instruction set, internally the processor did not directly execute System/3 instructions. To minimize cost, the I/O devices were very "dumb" and the processor (called the Control Storage Processor, or CSP) was optimized for low-level control of the I/O devices. The code executing in the CSP (called microcode) was contained in a writable control storage and included an emulator of the System/3 instruction architecture.

Microcode for the System/32 was much simpler than the typical "horizontal" microcode as implemented on other IBM processors. The System/32 processor had what we would later call a RISC architecture. Instructions were 16-bits wide and addresses were 16-bits wide. The microcode control storage was 4K 16-bit words (an extension of another 4K was available and contained emulation of floating-point arithmetic). Four usable interrupt levels each had eight 16-bit general purpose registers. All logical and arithmetic instructions were register-to-register operations. Main memory was accessed only by load and store instructions. There were condition flags for branching and only about 19 basic instructions. Of course, there was no pipelining, no caches, no branch prediction, or other performance features like are common on modern processors.

A CSP instruction executed in, on average, five to seven 200ns clock pulses. That is, a CSP microcode instruction instruction executed in about the same time as the 1.5us processing of a byte in the memory-to-memory architecture of the System/3. Since emulation of a System/3 architecture instruction tool many microcode cycles, emulation of System/3 was very slow. This lead to the innovation of moving some of the SCP into microcode (discussed more below).

Sidebar: My Personal History With the IBM System/32

In 1970, I finished my work on system software for the IBM System/3 Model 6. I then worked on the architecture of another proposed small computer, which was killed and the key personnel asked to move to Rochester MN. I was offered the job of the second line manager developing the operating system (the System Control Program, in IBM terminology) for a new small computer system (which ultimately became the System/32). So, in 9/71, I moved to Rochester (getting married to an IBM programmer on the way).

As well as being the manager of the system software development, I proposed and personally coded microcode that implemented some of the disk management functions of the operating system in microcode. While this seems obvious now, at the time I believe this was the first time this was done in IBM (moving SCP functions into microcode). I vividly remember watching the 1972 Olympics at home nights while also translating the System/3 SCP disk management code in microcode. There was no simulator of the CSP; the only way to debug code was on the hardware. Machine time was hard to get since there were few models available and the engineers were busy working on them. Thus, I mainly found time to use the hardware at night. I remember long nights sitting in front of a very early hardware model and debugging my code using the control panel shown in Figure 4 (the only debug mechanism available). Due to the developing nature of the hardware, I also found a few bugs in the hardware.

The Museum's System/32

The museum acquired the System/32 from its original owner in a small ranching and farming town in Oregon. This information came from its only owner before the museum acquired it:

"I bought it new in the 70's for a farm machinery business to do bookkeeping and some inventory control. There were no bookkeeping software packages tailored for the 32 at that time, so the IBM sales folks found me a system 3 package and a programmer to modify it to run on the 32. My training to this point was a degree in engineering using a slide rule. I worked with the programmer and the wonderful instruction manuals and learned RPGII and went on to write many programs for the 32 and my business."