[TUHS] Discuss of style and design of computer programs from a

Stephen Kitt steve at sk2.org
Sun May 7 17:42:27 AEST 2017


On Sat, 6 May 2017 21:45:19 +0000, Michael Kjörling <michael at kjorling.se>
wrote:
> On 6 May 2017 16:00 -0400, from usotsuki at buric.co (Steve Nickolas):
> > In 6502 code, it's not uncommon to do something like
> > 
> > foo1:     lda      #$00
> >           .byte    $2C       ; 3-byte BIT
> > foo2:     lda      #$01
> >            .
> >            .
> >            .
> > 
> > to save a byte (and probably still done for the few who write in
> > ASM). The "2C" operand would cause it to disassemble as something
> > like...
> > 
> > 1000-     LDA      #$00
> > 1002-     BIT      $01A9
> > 
> > which is the route you'd go down if you called "foo1".  Apart
> > diddling a few CPU flags, and an unneeded read on $01A9, harmless.  
> 
> You could do something quite similar on the 8086, which I am somewhat
> more familiar with.
[...]

One possible equivalent 16-bit x86 is

	foo1:	mov al, 0
		db 035h		; XOR AX, imm16
	foo2:	mov al, 1

which implements a branchless fall-through. (But you’d probably just use
SALC...)

> Or, something slightly more "useful"
> 
>         jmp short $+3
>         int 0 ; hex: cd 00
>         db 0
>         jz somewhere
> 
> which, again IIRC, would clear the accumulator (AX) register because
> 0000 hex is XOR AX,AX,

According to my disassembler 0000 is "add byte ptr [bx+si], al"; "xor ax, ax"
is either 31c0 or 33c0.

> which as a side effect sets the zero flag
> because the result is zero, and then executes a conditional jump if
> the zero flag is set (which it will be). In other words, it _executes_
> as
> 
>         jmp short l1
>         db 0cdh ; dummy byte
>     l1: xor ax,ax ; hex: 00 00
>         jz somewhere
> 
> but a naiive decompiler will see the CDh byte after the jump and take
> that as the first byte of the two-byte interrupt instruction. I don't
> know what 00 followed by the first byte of the JZ instruction would
> be, but probably a register/register XOR with a register store. Make
> it a far jump and you have a few extra bytes to play with, to befuddle
> anyone trying to figure out just what on Earth your code is really
> doing.

My memories of disassembling x86 code in the early 90s at least is that,
because of all this, disassemblers were pretty good at restarting streams
from jump destinations, so it never really was an issue. You need to get
creative with single-stepping mode and self-decrypting code to really annoy
people trying to understand your code ;-).

> The above, of course, is really just scratching the surface. With how
> many variable-length instructions the x86 has, with careful selection
> of opcodes and possibly use of undocumented but functionally valid
> combinations, I wouldn't be the least bit surprised if it's
> technically possible to write a 8086 program that does one thing when
> started normally, and something utterly different but still useful
> when started at an offset that results in execution beginning in the
> middle of an instruction. Bonus points if the first thing that program
> does is jump into itself _at that offset_. Now, the _work involved in
> doing so_, never mind maintaining it...

I have seen some short sections of code which do this. The x86 ISA is so rich
that you can make it adapt to many different constraints — one I rather liked
is assembling code which is valid ASCII (amaze your friends by typing in
a .COM using COPY CON!). There’s a C compiler which does that on GitHub
somewhere.

> Of course, I wouldn't do anything like the above in any real-world
> code base meant to run on modern systems unless obfuscation really
> _was_ a design goal. Honest. If I was tasked to write something that
> really needed to do something non-trivial in 16 KiB RAM on an original
> IBM 5150 PC or clone? I'd probably spend quite a bit of time in front
> of the whiteboard and with reference manuals before writing even one
> line of code...

Yes, and combine that with assemblers which output all possible encodings of
a given instruction!

Regards,

Stephen


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