Processor Register Size Bus size to RAM Max RAM size I/O Bus Comment
8086 16 16 1M XT First processor for PC. Not popular due to 16-bit bus to RAM
8088 16 8 1M XT Processor chosen by IBM for PC. Popular due to 8-bit bus to RAM. Older memory could be used with new system
80286 16 16 16M ISA Essentially the 8086, but with additional memory space. Protected mode is introduced.
80386 32 32 4G ISA (EISA,MCA) Full 32-bit bus to RAM. EISA and MCA 32-bit I/O buses are introduced, but not accepted widely.
80386SX 32 16 4G ISA (EISA,MCA) The 386 but with a data path to RAM of 16 bits. Done for the same reason as the 8088.
80486 32 32 4G ISA or PCI/ISA (EISA,MCA) More integration of the processor. The math coprocessor is added to the chip along with a cache and a pipeline.
80486SX 32 32 4G ISA or PCI/ISA (EISA,MCA) 486 with the math coprocessor disabled. Could buy another chip that would provide the coprocessor, it was actually a full 483, but only its coprocessor was used!
Pentium 32 32 4G PCI/ISA 2 pipelines.
Pentium Pro 32 32 4G PCI/ISA 2 level cache, split level 1 cache. Up to five instructions executed at a time.
Pentium II 32 32 4G PCI/ISA Pentium Pro with MMX
Pentium III 32 32 4G PCI/ISA Pentium II with new instructions for the internet.

Buses

XT
Original 4.77 MHz bus in the 8088. 8-bit I/O bus.
ISA
Original 8.33 MHz bus in the 80286. 16-bit I/O bus.
MCA
32-bit proprietary I/O bus designed by IBM.
EISA
A group of companies response to IBM. 32-bit non-proprietary 32-bit I/O bus. Still at 8.33 MHz, but wider than ISA.
PCI
Intels response to EISA and MCA. 32-bit I/O bus, later expanded to 64-bit. ISA bus can be added to the bus as just another device. Started at 33 MHz, later at 66 MHz.