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

[Steve Nickolas]

On Sat, 6 May 2017, Michael Kjörling wrote:

> On 6 May 2017 08:09 -0700, from corey at lod.com (Corey Lindsly):
>> Anyway, I reached one point in the assembly code that I simply could not
>> understand. It seemed like a mistake, and I went through it again and
>> again until I finally realized what it was doing. There was a branch/loop
>> that jumped to the middle of a multi-byte machine instruction, so that
>> branch had to be disassembled and stepped separately until it "synced" up
>> with the other branch again. Maybe this is standard practice in
>> programming (I don't know) but at the time I thought, what kind of evil
>> genius devised this to save a few bytes of memory?
>
> IIRC, that _was_ a common trick at least on machines of that class. It
> did have the potential to save a few bytes, yes (more if the
> instructions were such that you'd get some _other, desired_, behavior
> by jumping into the middle of one with some specific state), but it
> also foiled lots of disassemblers: Simply disassembling a binary from
> start to finish would yield nonsense in those locations, as you
> experienced. It thus basically forced you to single-step those
> instructions to figure out what was going on from the binary.
>
> I'm pretty sure it works on every architecture with variable-length
> instructions and arbitrary jump capability, as long as you have
> control over the specific machine instructions generated (such as if
> you are programming in assembler). Of course, it _is_ also a total
> nightmare to maintain such code.
>
> I would absolutely not say that doing something like that is standard
> practice in modern programming. Even in microcontrollers, where
> program and data memory can be scarce even today, I would argue that
> the costs would not outweigh the benefits by a long shot.

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.

(Most 6502 programmers would probably see a strange BIT instruction as an 
attempt to do this.)

It's probably not a good idea to still do this unless you're really REALLY 
crunched for space.

-uso.