tasm encoding

Tavis Ormandy

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Feb 22, 2021, 12:34:53 PM2/22/21

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Hello, I'm trying to convince TASM to generate the sign extending form
of and r16,imm8, something like this:

83 E7 FE and di, 0fffeh

I've tried all ways I can think of writing it (not 1, -2, etc),
but it always generates

81 E7 FE FF and di, 0FFFEh

Using "byte -2" seems to generate incorrect code, but maybe there's a
rationale I don't understand.

I thought perhaps a TASM expert knows a magic incantation or directive to
get the encoding I'm looking for? I'm not really a TASM programmer, I'm
trying to fix an old DOS driver.

Obviously I can just db it in there or use a macro, I'll do that if I
have to, but I think it might get messy.

Tavis.

--
_o) $ lynx lock.cmpxchg8b.com
/\\ _o) _o) $ finger tav...@sdf.org
_\_V _( ) _( ) @taviso

wolfgang kern

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Feb 22, 2021, 11:16:32 PM2/22/21

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On 22.02.2021 18:34, Tavis Ormandy wrote:
> Hello, I'm trying to convince TASM to generate the sign extending form
> of and r16,imm8, something like this:
>
> 83 E7 FE and di, 0fffeh
>
> I've tried all ways I can think of writing it (not 1, -2, etc),
> but it always generates
>
> 81 E7 FE FF and di, 0FFFEh
>
> Using "byte -2" seems to generate incorrect code, but maybe there's a
> rationale I don't understand.
>
> I thought perhaps a TASM expert knows a magic incantation or directive to
> get the encoding I'm looking for? I'm not really a TASM programmer, I'm
> trying to fix an old DOS driver.
>
> Obviously I can just db it in there or use a macro, I'll do that if I
> have to, but I think it might get messy.

it's several decades ago when I tried TASM, what I still remember is
that "db" was the only option to get desired opcodes.
your example is just one of a lot unsupported code variants in TASM.
__
wolfgang

Tavis Ormandy

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Feb 23, 2021, 12:00:45 AM2/23/21

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I thought that might be the answer, thanks anyway!

Alexei A. Frounze

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Feb 23, 2021, 3:41:20 AM2/23/21

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On Monday, February 22, 2021 at 9:34:53 AM UTC-8, Tavis Ormandy wrote:
> Hello, I'm trying to convince TASM to generate the sign extending form
> of and r16,imm8, something like this:
>
> 83 E7 FE and di, 0fffeh
>
> I've tried all ways I can think of writing it (not 1, -2, etc),
> but it always generates
>
> 81 E7 FE FF and di, 0FFFEh
>
> Using "byte -2" seems to generate incorrect code, but maybe there's a
> rationale I don't understand.

TASM 3.2 does it just fine. Here's a listing file I get:
----8<----
Turbo Assembler Version 3.2 02/22/21 10:58:12 Page 1
and.asm

1 ;.8086
2
3 0000 code segment use16
4 assume cs:code
5 org 100h
6
7 0100 start:
8 0100 83 E7 FE and di, 0fffeh
9 0103 83 E7 FE and di, -2
10 0106 83 E7 FF and di, byte -2 ; generates 0ffh instead of 0feh
11
12 0109 code ends
13 end start
...
----8<----

Throwing in .8086 didn't seem to make any effect, even though 083H is an encoding from a newer CPU, the 80386.

The 80286 knows these:
20 /r AND eb,rb 2,mem=7 Logical-AND byte register into EA byte
21 /r AND ew,rw 2,mem=7 Logical-AND word register into EA word
22 /r AND rb,eb 2,mem=7 Logical-AND EA byte into byte register
23 /r AND rw,ew 2,mem=7 Logical-AND EA word into word register
24 db AND AL,db 3 Logical-AND immediate byte into AL
25 dw AND AX,dw 3 Logical-AND immediate word into AX
80 /4 db AND eb,db 3,mem=7 Logical-AND immediate byte into EA byte
81 /4 dw AND ew,dw 3,mem=7 Logical-AND immediate word into EA word

The 80386 knows these:
20 /r AND r/m8,r8 2/7 AND byte register to r/m byte
21 /r AND r/m16,r16 2/7 AND word register to r/m word
21 /r AND r/m32,r32 2/7 AND dword register to r/m dword
22 /r AND r8,r/m8 2/6 AND r/m byte to byte register
23 /r AND r16,r/m16 2/6 AND r/m word to word register
23 /r AND r32,r/m32 2/6 AND r/m dword to dword register
24 ib AND AL,imm8 2 AND immediate byte to AL
25 iw AND AX,imm16 2 AND immediate word to AX
25 id AND EAX,imm32 2 AND immediate dword to EAX
80 /4 ib AND r/m8,imm8 2/7 AND immediate byte to r/m byte
81 /4 iw AND r/m16,imm16 2/7 AND immediate word to r/m word
81 /4 id AND r/m32,imm32 2/7 AND immediate dword to r/m dword
83 /4 ib AND r/m16,imm8 2/7 AND sign-extended immediate byte with r/m word
83 /4 ib AND r/m32,imm8 2/7 AND sign-extended immediate byte with r/m dword

So, either your TASM doesn't know the 80386 encodings or is somehow restricted to produce the pre-80386 ones.
HTH,
Alex

Tavis Ormandy

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Feb 23, 2021, 9:55:03 AM2/23/21

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On 2021-02-23, Alexei A. Frounze wrote:
>> 81 E7 FE FF and di, 0FFFEh
>>
>> Using "byte -2" seems to generate incorrect code, but maybe there's a
>> rationale I don't understand.
>

> So, either your TASM doesn't know the 80386 encodings or is somehow restricted to produce the pre-80386 ones.

Ah-ha, you're right - it was because I was using the NOSMART directive.

Unfortunately turning that off breaks a different encoding! I'll have to
experiment, hopefully it's an easy fix :)

Thank you.