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?
I have posted additional details here
<https://www.rehsdonline.com/post/troubleshooting-cpu-flags-register-between-bios-and-freedos>
, including a video walk-through of what I am seeing.
Thank you!
Rich
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