Hacker News Reader

Comment by SomeoneFromCA | Hacker News Reader

Contrary to what the others are saying, 8086 and 8088 have more differences than just bus size. Certain instruction have different timings, and as I vaguely remember, pipeline depth was also different.

View on HN

The effect of the 8-bit bus and prefetch queue are described in detail here:

http://www.jagregory.com/abrash-black-book/#the-8-bit-bus-cy...

The 8086 and 8088 CPUs differ essentially from one another by their respective data bus widths (the 8086 uses a 16-bit data bus, and the 8088 uses an 8-bit data bus). The Execution Unit for each processor is identical. The Bus Interface Unit for the 8086 incorporates a 16-bit data bus and a 6-byte instruction queue whereas the 8088 incorporates an 8-bit data bus and a 4-byte instruction queue.

See the iAPX 86,88 User's Manual http://www.bitsavers.org/components/intel/_dataBooks/1981_iA...

You contradict yourself, when you say "differ essentially from one another by their respective data bus widths" and "8086 incorporates a 16-bit data bus and a 6-byte instruction queue whereas the 8088 incorporates an 8-bit data bus and a 4-byte instruction queue.". In a sense 8088 has shorter pipeline. Shorter pipeline makes optimization techniques for 86 and 88 different, irrespective of the bus width - I mean, even if you slap 16 bit bus to 88, without changing the queue size, most efficient optimizations would still be different between these 2 CPUs.

I'm literally quoting from page 4-1 of the Intel 8086/8088 manual.

And how the fact that you are quoting from some manual contradicts the fact that speed is different because of different pipeline depth?

Reply on news.ycombinator.com