On 4/30/2023 2:53 PM, rehsd.i...@gmail.com wrote:
Hi, everyone. I’m new here. This is my first post. 😊
I am currently working on getting FreeDOS running on my homebrew 286
system. I am running into an issue where interrupt calls from FreeDOS
to my BIOS are not showing updated values for the CPU flags register
after the call. This happens with built-in interrupt calls in FreeDOS
or custom calls I am making. Here’s a simple test function that I
added to FreeDOS’s main.c (I call it just before signon()):
*void test_interrupt()*
*{*
* iregs regs;*
* regs.a.x = 0x1122;*
* regs.b.x = 0x0033;*
**
* regs.c.x = 0x4455;*
* regs.d.x = 0x6677;*
* regs.di = 0x8899;*
* regs.si = 0xaabb;*
**
*init_call_intr(0x15, ®s);*
**
* printf("\nInt test: ");*
*printf("ax=%04X bx=%04X cx=%04X dx=%04X di=%04X si=%04X
flags=%04X\n",regs.a.x,regs.b.x,regs.c.x,regs.d.x,regs.di,regs.si,regs.flags);*
*}*
In the assembly code for the BIOS interrupt service routine (I’m just
using interrupt 0x15 for testing), I update the flags register. When I
try to read the updated flags register back in FreeDOS, I am not
getting the values that I expect. For example, if I use something like
the following in my interrupt service routine, when I read regs.flags
in test_interupt() above, the carry flag may or may not be set
properly (same with the zero flag).
* mov ah, 0b01000001 ;SF, ZF, AF, PF,
and CF flags (bits 7, 6, 4, 2, and 0, respectively) – other flags are
left unchanged by CPU*
* sahf*
* iret*
In, kernel/pcb.h
<https://github.com/FDOS/kernel/blob/master/hdr/pcb.h>, I see some
notes about “offsets must match the assembly process” and a couple of
different layouts for the offsets. Could this be related to the issue
I am having? I am using NASM 2.16.01 and Watcom 1.9.
Suggestions or things I could look into?
Yes, check with the compiler you are using. And with the BIOS of your
system. Because nothing in that function is even remotely touching
(Free)DOS....
Ralf
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