On 5/7/14, 6:52 PM, Paolo Bonzini wrote:
Il 07/05/2014 14:32, Nadav Amit ha scritto:
32-bit operations are zero extended in 64-bit mode. Currently, the
code does
not handle them correctly and keeps the high bits. In 16-bit mode, the
high
32-bits are kept intact.
In addition, although it is not well-documented, when address override
prefix
is used with REP-string instruction, RCX high half is zeroed even if
ECX was
zero on the first iteration (as if an assignment was performed to ECX).
Is this true even for REPZ and ZF=0 or REPNZ and ZF=1?
Paolo
The REPZ and REPNZ condition is checked on the end of an iteration (see
the REP instruction description on the SDM), so it does not matter. So
even REPZ/RENZ would zero RCX high half.
This "feature" is not well-documented, but can be observed. Here is a
small code you can try.
---
#include <stdio.h>
unsigned long src, dst;
int main()
{
unsigned long long rsi, rdi, rcx;
rcx = 0xffffffff00000000ull;
rsi = (unsigned long long)&src | 0xffffffff00000000ull;
rdi = (unsigned long long)&dst | 0xffffffff00000000ull;
printf("before: rsi %llx rdi %llx rcx %llx\n", rsi, rdi ,rcx);
asm volatile ( ".byte 0x67\n\t"
"repne cmpsd\n\t"
: "+S" (rsi), "+D" (rdi), "+c" (rcx)
: : "memory", "cc" );
printf("after: rsi %llx rdi %llx rcx %llx\n", rsi, rdi ,rcx);
return 0;
}
---
Nadav
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