arch-x86: Fix real mode far jumps with set MSB in the offset.
When performing a real mode far jump, we were computing the offset into
the segment more or less correctly, but then when we tried to actually
set the PC using it, we used the second of the two wrip microop
arguments. The first argument is an unsigned value and is intended to be
a base to work from when figuring out the new IP, and the second
argument is a signed offset which can be used to implement relative
jumps/branches. When we used the second operand for our new value,
setting the first operand to t0 (the zero register on x86), we would
inadvertantly sign extend it since the wrip instruction would treat it
as a signed value.
Instead, we can just switch the two operands so that the wrip microop
treats the desired value as the unsigned base, and then adds a signed t0
to it, which will still be 0 one way or the other.
Also, while researching this bug, I found that the size used for
computing the new IP is always the operand size, and never the address
size. This CL fixes that problem as well by removing the faulty
override.
Change-Id: I96ac9effd37b40161dd8d0b634c5869e767a8873
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/55243
Reviewed-by: Gabe Black <gabe.black@gmail.com>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>
1 file changed
