IBM PC/XT Architecture Overview
The design of the IBM 5150 Personal Computer reflects IBM’s project requirements to create a low-cost, maintainable system capable of expansion.
CPU
IBM chose the Intel 8088 for the 5150. The 8088 was a lower-cost variant of the 8086 CPU. While still 16-bit internally, the 8088 only had an 8-bit data bus. This simplified the 5150’s motherboard design, and made it easy to build a system around Intel’s various 8-bit peripheral chips.
The 8088 has 20 address lines, allowing it to address \(2^{20}\) bytes, or 1MB.
IBM chose to reserve addresses above 0xA0000, leading to the infamous “640KB” memory limit that is often mistakenly blamed on Microsoft.
Expansion Bus
The 8-bit data bus of the 8088 would also dictate the 8-bit data width of the system’s expansion bus. This bus would later be expanded to 16-bits with the IBM 5170 AT, and would later be dubbed the ISA bus by Compaq1 and a growing consortium of PC clone manufacturers.
System Clock
The 5150 has a single main system crystal with a frequency of 14.31818MHz. This frequency is exactly four times the NTSC color subcarrier frequency.
The crystal frequency can be expressed as a fraction:
$$f_{crystal} = \frac{315}{22} \text{ MHz} = 14.318181\overline{81} \text{ MHz}$$
This choice was made to make the PC more easily compatible with North American television sets, making available a low-budget display option. This may seem like an odd choice for a business-oriented computer, but it allowed the IBM Color Graphics Adapter to omit a separate crystal.
The CPU frequency (4.77 MHz) is obtained by dividing the system clock by 3:
$$\frac{14.3181818}{3} = 4.773MHz$$
The 8088 was rated for 5MHz operation2, so this represents about a 5% underclock.
References
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wikipedia.org Industry Standard Architecture. ↩
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Intel Corporation. 8088 8-Bit Hmos Microprocessor. Intel Corporation, August 1990. Document Number: 231456-006. Available at: Intel 8088 Data Sheet PDF ↩