Newbie member saying something!!!

From: allisonp_at_world.std.com <(allisonp_at_world.std.com)>
Date: Tue Sep 7 15:58:32 1999

I don't think that works as the prefetch is coherent. To my experience
the que was nearly transparent to programs and I never allowed for it
presence not incurred any difficulty for that offense. It's fuzzy but
branches and several other events dump the queue.

Allison


On Tue, 7 Sep 1999, Sean 'Captain Napalm' Conner wrote:

> It was thus said that the Great Tony Duell once stated:
> >
> > >
> > > Nope, it is the siutcase style machine, with a 8088 processor (I checked in
> >
> > The 8088 and 8086 are actually very similar processors. The main
> > difference is the external data bus width -- the 8086 has a 16 bit
> > external bus and the 8088 has an 8 bit external bus. I seem to remember
> > that the internal instruction queue is a different size on the 2 chips
> > but I would have to check.
>
> 8086 has a 6-byte prefetch queue while the 8088 has a 4-byte prefetch
> queue. I did a *lot* of 8088 assembly programming years ago. Possibly too
> much 8-)
>
> > It is therefore difficult to tell the chips apart in software and a lot
> > of 'machine info' type utilities report finding an 8088 even on
> > 8086-based machines.
>
> I don't think it's that difficult.
>
> org 0100h
> mov al,0 ;[1]
> mov bx,offset jmp_point + 1
>
> mov [bx],al ;[2]
> nop ;[3]
> nop
> nop ;[5]
> nop
> jmp_point: jmps x86_chip ;[4]
>
> mov dx,offset t88_chip
> jmps display
>
> x86_chip: mov dx,offset t86_chip
>
> display: mov ah,9
> int 21h
>
> t88_chip db 'Hi, we're an 8088!',13,10,'$'
> t86_chip db 'Hi, we're an 8086!',13,10,'$'
>
> [1] We're going to change the offset of the jump at point [3] to
> 0. On an 8088, this will take effect since the instruction will
> not have been read in by the time we write the modification at
> point [2]. On an 8086, the instruction at point [4] will already
> be in the pre-fetch queue and won't be modified (well, it will be
> modified in memory, but not in the pre-fetch queue).
>
> [2] This is a two byte instruction. After the primary byte is read, the
> 8086 will be fetching the first byte of the instruction at point
> [4], while on an 8088, it will be reading in the instruction at
> point [5]. On the second byte, the 8086 will have finished reading
> the JMP instruction, while the 8088 will read the next NOP. The
> write back will happen at JMP_POINT+1, which the 8086 has already
> read, while the 8088 has yet to read.
>
> [3] Filler space for the 8088 4-byte prefetch queue.
>
> [4] The instruction we're going to modify to determine if we're on an
> 8086 or 8088.
>
> [5] See note [2].
>
> > IMHO the only reliable way to know what CPU you have is to read the
> > markings on the chip package.
>
> Ah, but a program will have a problem doing that.
>
> -spc (Note: this is untested code. Theoretically, it should work, but I
> don't have an 8086 to test it on)
>
Received on Tue Sep 07 1999 - 15:58:32 BST