Re: 80286 protected mode

On Sat, 12 Oct 2024 18:17:18 +0000, Brett wrote:
 

Brian G. Lucas <bagel99@gmail.com> wrote:

On 10/12/24 12:06 AM, Brett wrote:

MitchAlsup1 <mitchalsup@aol.com> wrote:

On Fri, 11 Oct 2024 12:10:13 +0000, David Brown wrote:
 

 
Do you think you can just write this :
 
void * memmove(void * s1, const void  * s2, size_t n)
{
return memmove(s1, s2, n);
}
 
in your library's source?

 
.global memmove
memmove:
MM     R2,R1,R3
RET
 
sure !
 

 
Can R3 be a const, that causes issues for restartability, but branch
prediction is easier and the code is shorter.
 

Yes.
#include <string.h>
 
void memmoverr(char to[], char fm[], size_t cnt)
{
memmove(to, fm, cnt);
}
 
void memmoverd(char to[], char fm[])
{
memmove(to, fm, 0x100000000);
}
Yields:
memmoverr:                              ; @memmoverr
mm r1,r2,r3
ret
memmoverd:                              ; @memmoverd
mm r1,r2,#4294967296
ret

 
Excellent!
 

Though I guess forwarding a const is probably a thing today to improve
branch prediction, which is normally HORRIBLE for short branch counts.

 
What is the default virtual loop count if the register count is not
available?

 
There is always a count available; it can come from a register or an
immediate.
 

Worst case the source and dest are in cache, and the count is 150 cycles
away in memory. So hundreds of chars could be copied until the value is
loaded and that count value could be say 5.

 
The instruction cannot start until the count in known. You don't start
an FMAC until all 3 operands are ready, either.

Lots of work and time
discarded, so you play the odds, perhaps to the low side and over
prefetch to cover being wrong.