MS-DOS writes 0 bytes if an entire write cannot be fulfilled due to disk space

[ecm-pushbx]

I just added proper disk full checks to every (non-stdout) file write in WarpLink: https://hg.pushbx.org/ecm/warplink/rev/6fe1ccef6b07

I noticed that there were some checks already, but they only checked for ax returned as exactly zero. Turns out that this is sufficient on MS-DOS (tested current lMS-DOS, MS-DOS v5.00, and MS-DOS v7.10), as they always appear to return either the full ax equal to cx, or ax as zero if there isn't enough disk space.

The interrupt list only states that "the usual cause for AX < CX on return is a full disk", but it doesn't say that the return is never 0 < ax < cx. So that's probably why FreeDOS behaves differently.

[ecm-pushbx]

Enhanced DR-DOS also appears to return zero, not doing partial writes.

[ecm-pushbx]

Test case, testspac.asm:

	section DAT1
times 4 * 1024 db 26h
	global labellabellabellabel1
	global labellabellabellabel2
	global labellabellabellabel3
	global labellabellabellabel4
	global labellabellabellabel5
	global labellabellabellabel6
	global labellabellabellabel7
	global labellabellabellabel8
	global labellabellabellabel9
	labellabellabellabel0
	labellabellabellabel1
	labellabellabellabel2
	labellabellabellabel3
	labellabellabellabel4
	labellabellabellabel5
	labellabellabellabel6
	labellabellabellabel7
	labellabellabellabel8
	labellabellabellabel9
	labellabellabellabel0

The labels are for filling the .map file if you choose to create one. Assemble as nasm -f obj testspac.asm

The script file testspac.bat:

ldebug /t /p /c="bp at ptr ri21p when ah==40 && bx != 1" wl%1 testspac.obj,a:testspa8.exe;

Prepare the diskette file system in drive A: to have 3072 bytes free (6 clusters), not enough for the full resulting .exe file. Then run the batch file, and repeatedly run g commands to the debugger. Older WarpLink revisions on current FreeDOS won't detect the disk full condition on the third and final write to the executable file, as the kernel will do a partial write so ax is nonzero on return to the application. lMS-DOS and MS-DOS will let older WarpLink detect the disk full as the 4096 byte write attempt returns ax as zero.