Question; it's been 20 years since I've done assembler, but I want to understand...

acqq · on June 7, 2019

> - The downloaded COM file is almost 64k

I don't know how you did that, but now I've downloaded it and I get:

    ls -l invaders.com
    -rwxrwxrwx 1 user group  518 Jun 7 00:00 invaders.com

The boot sector version is:

    ls -l invaders.img
    -rwxrwxrwx 1 user group  512 Jun 7 00:00 invaders.img

The .com file is bigger because it has "more features" -- it allows one to "exit" the game, and in the com version it doesn't have to be less or equal of 512 bytes to fit, the boot sector version has that limitation.

I also see that even the first commit had both the .com and the .img 512 bytes:

     9e7101f
     BIN +512 Bytes invaders.com 
     BIN +512 Bytes invaders.img

I was also able to configure a virtual machine to boot from invaders.img (512 bytes) as a "floppy disk image."

300bps · on June 7, 2019

in what sense is this "512 bytes"?...The source code appears to be about 4k

The source .asm file is about 4k, most of which consists of comments.

The rest is assembly language code with things like:

        mov ah,al

That assembly language which is 17 bytes including the preceding spaces gets assembled into machine language which is substantially lower. I'm much more familiar with 6510 assembly than I am 8086 assembly but I believe mov ah,al is one byte for the mnemonic and two bytes for the registers for a total of 3 bytes. So the actual machine code is far smaller than the assembly source code.

acqq · on June 7, 2019

> I believe mov ah,al is one byte for the mnemonic and two bytes for the registers for a total of 3 bytes.

Two bytes in the 8086 mode:

     88C4   mov ah,al

And inside, ah and al are encoded with the 3 bits each, spending just 6 bits for "from which register to which register."

a1369209993 · on June 7, 2019

> mov ah,al is

One byte for [mov Gb Eb] and one byte for [modrm sp ax] (ah == (byte) r4 (word) == sp). x86 is pretty good about code density.

bepvte · on June 7, 2019

Compiled, it is 512 bytes. The com file is for running out side of the boot sector afaik.

simcop2387 · on June 7, 2019

It's also possible that the com file is just being padded because of the way they work, statically loaded into a specific 64kb segment of ram, limiting their size to 64kb. The assembler might just be padding it to zero out the rest of the segment, even though it isn't needed.

oops · on June 7, 2019

Or the OP downloaded the HTML page[1] instead of the raw file. The HTML for a signed-out user is about 64kB.

1: https://github.com/nanochess/Invaders/blob/master/invaders.c...

acqq · on June 7, 2019

I never saw any unnecessary padding there. The initial commit was 512 bytes for .com.

     9e7101f
     BIN +512 Bytes invaders.com

One can also boot directly from the 512 byte floppy disk image .img, e.g.

    qemu-system-i386w -fda invaders.img