Re: [Freedos-devel] CPU flags register not updating on interrupt calls
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, );*
**
*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, 0b0101 ;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
, 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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Re: [Freedos-devel] CPU flags register not updating on interrupt calls
I ended up with "retf 2" and it seems to be working fine. I will see if I can
switch out the retf 2 with stack frame manipulation, like you show.
Thanks!
Rich
-Original Message-
From: C. Masloch
Sent: Monday, May 1, 2023 3:31 AM
To: freedos-devel@lists.sourceforge.net
Subject: Re: [Freedos-devel] CPU flags register not updating on interrupt calls
On at 2023-04-30 17:19 -0500, rehsd.i...@gmail.com wrote:
> Not sure if it’s bad etiquette to reply to your own post, but… I may
> just need to change out my “iret” with “ret 2”. I’m doing some
> testing…
>
> Rich
In NASM you can indeed use this as an alternative, but you need *far* return,
that is "retf 2". However, I do not recommend this choice.
Instead, you can use a lahf or rcr \ rol trick to pass back either all
arithmetic status flags (excepting the Overflow Flag) or only the Carry Flag.
The core of your problem is that you were trying to set the current flags *in
the interrupt handler*. But once you run an iret instruction, it pops the flags
off the interrupt return stack frame. So to return flags to your caller, you
either have to go with retf 2 (with all its problems) or modify the flags image
in the stack frame before you run iret.
I recently commented on similar problems in [1] and [2]. I also gave an example
from my SEEKEXT tool [3]:
.iret_CF:
push bp
mov bp, sp
rcr byte [bp + 6], 1; flip
rol byte [bp + 6], 1; flop
pop bp
iret
One of the reports [2] also contains an lahf example:
push bp
mov bp, sp
push ax
lahf
; bp + 0 = saved bp
; bp + 2 = ip
; bp + 4 = cs
; bp + 6 = fl
mov byte [bp + 6], ah
pop ax
pop bp
iret
[1]: https://github.com/640-KB/GLaBIOS/issues/20
[2]: https://github.com/MobyGamer/softhddi/issues/1
[3]: https://hg.pushbx.org/ecm/seekext/file/bbfcfa0d1c0b/resident.asm#l254
Regards,
ecm
> *From:* rehsd.i...@gmail.com
> *Sent:* Sunday, April 30, 2023 4:53 PM
> *To:* freedos-devel@lists.sourceforge.net
> *Subject:* CPU flags register not updating on interrupt calls
>
> 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, );*
>
> **
>
> *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, 0b0101 ;SF, ZF, AF, PF,
> and CF flags (bits 7, 6, 4, 2, haveand 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-b
> etween-bios-and-freedos>, including a video walk-through of what I am
> seeing.
>
> Thank you!
>
> Rich
>
>
>
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>
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Re: [Freedos-devel] CPU flags register not updating on interrupt calls
That was me. I openend an issue in the FreeDOS Archive tracker thas OMF output seems to be broken in 2.16: https://gitlab.com/FreeDOS/devel/nasm/-/issues/1 I also mailed H.P. Anvin and informed him about what I consider to be a bug. I do not know if they have / will fix it, or if it is a misunderstanding by me. Greetings, Bernd > Am 01.05.2023 um 08:45 schrieb Rugxulo : > > Hi, > > On Sun, Apr 30, 2023 at 4:54 PM wrote: >> >> In, kernel/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. > > Someone reported that there was a regression in OMF/OBJ for NASM > 2.16.01 regarding 16-bit segments. > > Try reverting to NASM 2.15.05 instead: > > * > http://www.ibiblio.org/pub/micro/pc-stuff/freedos/files/devel/asm/nasm/2.15.05/ > > Or, for really having fun, try the ancient 0.98.39 16-bit version: > > * > http://www.ibiblio.org/pub/micro/pc-stuff/freedos/files/devel/asm/nasm/0.98.39/8086host/ > > > ___ > Freedos-devel mailing list > Freedos-devel@lists.sourceforge.net > https://lists.sourceforge.net/lists/listinfo/freedos-devel ___ Freedos-devel mailing list Freedos-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/freedos-devel
Re: [Freedos-devel] CPU flags register not updating on interrupt calls
On at 2023-05-01 01:45 -0500, Rugxulo wrote: Hi, On Sun, Apr 30, 2023 at 4:54 PM wrote: In, kernel/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. Someone reported that there was a regression in OMF/OBJ for NASM 2.16.01 regarding 16-bit segments. Try reverting to NASM 2.15.05 instead: * http://www.ibiblio.org/pub/micro/pc-stuff/freedos/files/devel/asm/nasm/2.15.05/ Or, for really having fun, try the ancient 0.98.39 16-bit version: * http://www.ibiblio.org/pub/micro/pc-stuff/freedos/files/devel/asm/nasm/0.98.39/8086host/ Please point to the exact bug report if you make claims like this. It wasn't actually anything to do with such a regression however. Regards, ecm ___ Freedos-devel mailing list Freedos-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/freedos-devel
Re: [Freedos-devel] CPU flags register not updating on interrupt calls
On at 2023-04-30 17:19 -0500, rehsd.i...@gmail.com wrote:
Not sure if it’s bad etiquette to reply to your own post, but… I may
just need to change out my “iret” with “ret 2”. I’m doing some testing…
Rich
In NASM you can indeed use this as an alternative, but you need *far*
return, that is "retf 2". However, I do not recommend this choice.
Instead, you can use a lahf or rcr \ rol trick to pass back either all
arithmetic status flags (excepting the Overflow Flag) or only the Carry
Flag. The core of your problem is that you were trying to set the
current flags *in the interrupt handler*. But once you run an iret
instruction, it pops the flags off the interrupt return stack frame. So
to return flags to your caller, you either have to go with retf 2 (with
all its problems) or modify the flags image in the stack frame before
you run iret.
I recently commented on similar problems in [1] and [2]. I also gave an
example from my SEEKEXT tool [3]:
.iret_CF:
push bp
mov bp, sp
rcr byte [bp + 6], 1; flip
rol byte [bp + 6], 1; flop
pop bp
iret
One of the reports [2] also contains an lahf example:
push bp
mov bp, sp
push ax
lahf
; bp + 0 = saved bp
; bp + 2 = ip
; bp + 4 = cs
; bp + 6 = fl
mov byte [bp + 6], ah
pop ax
pop bp
iret
[1]: https://github.com/640-KB/GLaBIOS/issues/20
[2]: https://github.com/MobyGamer/softhddi/issues/1
[3]: https://hg.pushbx.org/ecm/seekext/file/bbfcfa0d1c0b/resident.asm#l254
Regards,
ecm
*From:* rehsd.i...@gmail.com
*Sent:* Sunday, April 30, 2023 4:53 PM
*To:* freedos-devel@lists.sourceforge.net
*Subject:* CPU flags register not updating on interrupt calls
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, );*
**
*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, 0b0101 ;SF, ZF, AF, PF, and
CF flags (bits 7, 6, 4, 2, haveand 0, respectively) – other flags are left
unchanged by CPU*
* sahf*
*iret*
In, kernel/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
,
including a video walk-through of what I am seeing.
Thank you!
Rich
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Re: [Freedos-devel] CPU flags register not updating on interrupt calls
Hi, On Sun, Apr 30, 2023 at 4:54 PM wrote: > > In, kernel/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. Someone reported that there was a regression in OMF/OBJ for NASM 2.16.01 regarding 16-bit segments. Try reverting to NASM 2.15.05 instead: * http://www.ibiblio.org/pub/micro/pc-stuff/freedos/files/devel/asm/nasm/2.15.05/ Or, for really having fun, try the ancient 0.98.39 16-bit version: * http://www.ibiblio.org/pub/micro/pc-stuff/freedos/files/devel/asm/nasm/0.98.39/8086host/ ___ Freedos-devel mailing list Freedos-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/freedos-devel
Re: [Freedos-devel] CPU flags register not updating on interrupt calls
Not sure if it’s bad etiquette to reply to your own post, but… I may just need
to change out my “iret” with “ret 2”. I’m doing some testing…
Rich
From: rehsd.i...@gmail.com
Sent: Sunday, April 30, 2023 4:53 PM
To: freedos-devel@lists.sourceforge.net
Subject: CPU flags register not updating on interrupt calls
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, );
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, 0b0101 ;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 , 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
, including a video walk-through of what I am seeing.
Thank you!
Rich
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