* Re: Synthetic register related: maximum value of a modrm byte in i386
@ 2003-01-04 11:37 Robert Dewar
2003-01-04 14:26 ` Andy Walker
0 siblings, 1 reply; 3+ messages in thread
From: Robert Dewar @ 2003-01-04 11:37 UTC (permalink / raw)
To: gcc, ja_walker
> The next, and obvious, question is how negative can it be?
obvious indeed :-)
It is just normal twos complement notation, like any other integer on
the machine.
^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: Synthetic register related: maximum value of a modrm byte in i386
2003-01-04 11:37 Synthetic register related: maximum value of a modrm byte in i386 Robert Dewar
@ 2003-01-04 14:26 ` Andy Walker
0 siblings, 0 replies; 3+ messages in thread
From: Andy Walker @ 2003-01-04 14:26 UTC (permalink / raw)
To: Robert Dewar, gcc
On Saturday 04 January 2003 05:37 am, Robert Dewar wrote:
> > The next, and obvious, question is how negative can it be?
>
> obvious indeed :-)
>
> It is just normal twos complement notation, like any other integer on
> the machine.
Thank you very much.
Andy
^ permalink raw reply [flat|nested] 3+ messages in thread
* Synthetic register related: maximum value of a modrm byte in i386
@ 2003-01-04 6:53 Andy Walker
0 siblings, 0 replies; 3+ messages in thread
From: Andy Walker @ 2003-01-04 6:53 UTC (permalink / raw)
To: gcc
After much thought and reflection, and re-reading the Intel x86 Instruction
manual, I have changed my mind and decided to use the edp, or
hard-frame-register as the base register for the Synthetic regs.
Some modrm instructions on the x86 can use a single byte as the offset, and
it may be negative. In other words, the integer in the single byte is
signed.
The next, and obvious, question is how negative can it be?
I presume that B'0000 0000' represents zero.
I presume that B'0111 1111' represents 127.
I presume that B'1000 0001' represents -127.
What does B'1000 0000' represent?
-128, or +128 ?
My Fine Manual is not at all clear.
Andy
^ permalink raw reply [flat|nested] 3+ messages in thread
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