General Coding Tricks

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Revision as of 16:37, 12 August 2016 by HellMood (talk | contribs) (Accessing the timer tick for free)

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Data is code, code is data

Code is nothing more than data that the CPU interprets. For example, consider this multi-byte instruction:

        mov ah,37h

This assembles to B4 37. B4 by itself isn't interesting, but 37 happens to be the opcode for AAS. Let's say you had this code before a loop, and you needed to perform AAS at the top of a loop. Rather than put AAS at the top of the loop, you can reuse the opcode that will already be there as part of the mov ah,37 that comes before it. Just jump directly into the middle of the mov ah,37h, which will get interpreted and executed as AAS:

        mov ah,37h
        ;misc. stuff
        loop label+1

The +1 specifies the jump should go to 1 byte past the actual location.


You can use opcodes hidden in your existing data. For example, .COM files can end with RET, which is opcode C3. If you already have a C3 somewhere else in your code, even as part of data, just JMP to that pre-existing C3 instead of adding a RET.

If your environment holds you back, change it

The default MCGA palette is fairly horrible, but can be size advantages to changing it: While setting a new palette costs bytes, the new palette arrangement could save you headaches down the road. For example, if your code is calculating pixel colors that fall into goofy ranges, rather than constantly adjust the colors to sane ranges (ie. aligned to powers of 2), just set the palette so that values falling into those ranges look the way you want. (This assumes you have very small ways of redefining the palette, of course.)

The above is maybe not the best example. Rewrites to this section are welcome.

Need a constant?

If you need a constant value but you're out of space, search your assembled code for a byte value you can use.

A smaller way to point to Mode 13's screen segment

Rather than mov ah,0a0h; mov es,ax or push word 0a000h; pop es, try this 2-byte wonder:

les bp,[bx]

This sets ES=9FFF, only one away from A000. You can write to the screen with ES: this way as long as you are aware the segment is one paragraph (16 bytes) behind, so just increase your offset by 16 if you need exact placement.

How does this work? At start of execution of a .COM file, BX=0, and DS=CS. The contents of the COM file get loaded to offset 0x100 in that segment, but loaded before that is the PSP (program segment prefix), which DOS populates with information about the loaded program and other info. The PSP starts with CD 20 (INT 20, which exits the program), so that's what gets loaded into BP. The next word is the number of the last free conventional memory segment, typically 0x9fff (but can be something different if parts of the upper memory range are either not installed or allocated).

Warning: This trick doesn't always work. On FreeDos, this can set ES=9FE0 and there is something resident at that location that can screw up the system after normal program exit if you overwrite it.

Accessing the timer tick for free

If using a 386+, FS=0 at .COM start. So, FS:[046C] gets you the DOS timer tick variable, which you could use for timing/pacing, or a random seed. Some environments, especially EMS/XMS programs, can modify the FS register, so it can't always assumed to be 0000h. POP DS right after the start and accessing [046C] does the trick then with equal size.

Looping twice

If you need to repeat a section of code that doesn't modify the carry flag, and you know the carry flag is clear, you can loop once in only 3 bytes:

        ;do stuff here
        jc      looping