Gregory's ServerIntel's 8088 processor powered the IBM PC (1981), ensuring the continuing success of the x86 architecture. The PC team selected the 8088 largely because its system bus was similar to the Intel 8085 processor, which the team had used in the now-forgotten Datamaster computer. 1/8
Ken Shirriff
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Ken Shirriff
The Datamaster influenced the PC in many ways. Both desktop computers used BASIC. The PC used the same keyboard, but detached from the unit. The PC's 62-pin expansion bus came from the Datamaster. 3/8
Ken Shirriff
The Datamaster had an 80×24 display, matching IBM's popular 3270 terminal. (80 columns date back to punch cards.) The IBM PC was going to copy the Datamaster's display, but they managed to squeeze in one more line. That's why today's console windows default to 80×25. 4/8
Ken Shirriff
The Datamaster in my photos is at the System Source computer museum in Hunt Valley MD, just outside Baltimore. The museum is well worth a visit if you're in the area. 5/8
Ken Shirriff
Getting back to the 8088's system bus, the bus uses a four-step sequence of operations to access memory or I/O devices. These steps are called T-states: T1, T2, T3, and T4. A read operation provides the address during T1 and receives the data during T3. 6/8
Ken Shirriff
A complicated state machine moves the processor through the T-states. This diagram shows the location of the bus state machine circuitry on the die of the 8088 processor. Back in 1979, just a few flip-flops and logic gates occupied a substantial area on the die. 7/8
Ken Shirriff
The 8088's bus state machine is too complicated to explain here. Instead, see my blog post for lots of diagrams and schematics: https://www.righto.com/2024/04/intel-8088-bus-state-machine.html 8/8 
Chartreuse
@kenshirriff If anything that also shows just how optimized the rest is to not fit all the functionality in the rest of the space, especially with the decode logic taking up so much too. 
Tim Ward ⭐🇪🇺🔶 #FBPE
@kenshirriff Very sad that was, when at the time the 68000 family was so much better. (From the POV of the assembly language coder, anyway.)
mort666
@kenshirriff Funnily British Telecom (now BT) as it was called also used 8088 and 8086 as the CPU at the core of their System-X telephone system that replaced their old Strowger (SXS) switching system and telephone exchange gear. Back in the day, BT Engineering and R&D types had a public Engineering publication that in a multi-issue series the entire design to a circuit level for System-X, plus the x86 ASM source for the OS was published. The OS even had a version vuln to the old 8088 INT bug. 
spmatich :blobcoffee:
@kenshirriff it seems strange to think that it wasn’t until the i486 (1989) that this line of CPUs had on-chip cache. But suppose I am comparing an 8088 CPU with 28k transistors to one with over 1.1m 
David Penington
@kenshirriff The 8 bit bus on the Intel 8088 made the IBM PC compatible with the peripherals available for existing 8 bit PCs. DEC even introduced a "Rainbow" with both an 8 bit CPU and an 8088 in the same PC, so it could run both old & new software. 
Spicy Potato
@kenshirriff Thank you for these threads. They are always so fun and interesting to read. |
The Datamaster, also known as the IBM System/23, was released in July 1981, just a month before the IBM PC. The 16-bit PC overshadowed the 8-bit Datamaster as it was both more powerful and cheaper. But the Datamaster is interesting since it is the IBM PC's parent. 2/8