.COM register initialization

category: code [glöplog]

I'm trying to optimize a 38b .COM file down and for the life of me can't remember what .COM file register initializations are considered "safe". Looking at a debugger on my test rig, it APPEARS that this is true at .COM file startup:

ES=DS=SS=CS
IP=0100h
AX=BX=CX=DX=BP=SI=DI=0000h
Flags=no carry, no zero, no parity

Is this "reliable" or am I making too many assumptions?

I'm rusty; the only size tricks I can remember are:

RET can be used as a 1-byte "exit program"
PUSH reg1/POP reg2 is a 2-byte "copy from reg1 to reg2"
STOSW as a 1-byte text char output

If there is a pouet thread I should be consulting, please kick me (although I searched before posting this).

added on the 2012-12-13 23:37:54 by trixter

was i the only one thinking about the .com TLD first when reading the title? ;D

PS:
I have no idea.

added on the 2012-12-14 01:04:44 by wysiwtf

some informations about initial values can be found here:

link me beautiful

and I think it is okay to make assumptions. there are many intros doing that. sure there might be machines where it won't run but that can happen to every program. assumptions are no guarantee - but they saves bytes :D

added on the 2012-12-14 07:35:04 by sensenstahl

Fantastic link, thanks! That's exactly what I was looking for.

If I break the 32b barrier, I'll probably blog about it and post back here.

added on the 2012-12-14 17:23:29 by trixter

but don't forget that in 32b no one cares about ESC and clean exit (=

added on the 2012-12-14 17:28:05 by sensenstahl

You might also take a look at the specifications of the Hugi Size Coding Competitions, especially the last competitions.

Link: http://www.hugi.scene.org/compo/

added on the 2012-12-14 19:25:18 by Adok

Quoting from the specifications of the last compo (#29, general.txt):

Code:

You may assume that
  ... the registers have these values (all in hex):
      (xx - means an unknown value which MUST NOT be assumed)

          EAX = xxxx****
                AL = 00 if first FCB has valid drive letter,  FF if not
                AH = 00 if second FCB has valid drive letter, FF if not
          EBX = xxxx****
                BL = 00 if first FCB has valid drive letter,  FF if not
                BH = 00 if second FCB has valid drive letter, FF if not
          ECX = xxxx00FF
          EDX = xxxxxxxx
  DX  = CS = DS = ES = SS = xxxx, 0080 <= DX <=9000.
          ESI = xxxx0100
          EDI = xxxxFFFE
          EBP = xxxx09xx
          ESP = xxxxFFFE
          EIP = xxxx0100

  EFLAGS (binary) = xxxxxxxx xxxxxxxx xxxxx01x xx0x0x1x
      i.e.
          DF = 0
          IF = 1
          other flags = x

          WORD [FFFE] = 0000
          Layout of PSP: see [Memory Layout]

  ... that the program is not loaded high.
  ... that DPMI services are available.
  ... FCB functions can be used to access files whose name is given in 8.3
      format.

You must NOT assume that
  ... the  FPU  state is  defined;  you have to use  FINIT  to  initialize it
      before using it.

added on the 2012-12-14 19:27:10 by Adok

I ended up breaking the 16b barrier :-D

added on the 2012-12-14 20:19:31 by trixter

Now I am _really_ curious about what you are doing :)

added on the 2012-12-14 21:02:49 by Preacher