Comment by adrian_b | Hacker News Reader

Comment by adrian_b | Hacker News Reader

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This difference between 8088 and 8086 was used by several MS-DOS programs to detect the type of the CPU on which they were run.

It was easy to measure the length of the prefetch queue by a self-modifying program, which replaced the instruction placed 4 bytes after the current instruction pointer.

On an 8088, the new instruction was executed, while on an 8086 the old instruction was executed.

Various other such obscure differences (e.g. the value of a pushed stack pointer or the result of a shift operation with a shift count greater than the register size) were used to identify NEC V20 and V30, Intel 80186, 80188, 80286 and 80386, because architectural identification means, like the CPUID instruction, have been introduced only in Intel Pentium and in the late 486 models that were launched after Pentium.

> It was easy to measure the length of the prefetch queue by a self-modifying program

It became hard on the 386, as linked in the article: http://www.rcollins.org/secrets/PrefetchQueue.html

Official Intel program to detect the current processor (starts on page 16): http://kib.kiev.ua/x86docs/Intel/AppNote485/241618-005-mn.pd...

It may be official, but relying on undefined flags can fail: http://www.rcollins.org/ddj/Sep96/Sep96.html http://www.rcollins.org/ddj/Nov96/Nov96.html

For detecting 8086(/88) and NEC V30(/20) vs. 80186 and anything later, I'd recommend using a shift instruction with a count of 33. The 186 and later processors will mask it with 0x1F, resulting in a shift by 1 instead. A shift count of 32 would also behave differently, but not set any flags because a shift by 0 effectively becomes a NOP.

This is true even on x86-64, only for 64 bit operands is the count masked with 0x3F.

The P6 family was when Intel decided to add circuitry to detect SMC and flush the queue, effectively making it appear to have a queue size of 0. AFAIK all x86 henceforth have been the same.

I tried to track this down but I found sources claiming this was added to the 486, Pentium, and Pentium Pro (P6). So if you know for sure that it's P6, please let me know the details.

https://news.ycombinator.com/item?id=33928876

Coincidentally a topic I did some reseach into recently. It appears to be in both the Pentium(P5/54) and PPro(P6), but not the 486.

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