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trip report: Tiny C Compiler 0.9.27 (Miscellaneous)

posted by bretjohn Homepage E-mail, Rio Rancho, NM, 05.02.2026, 22:24

> I cannot remember all details currently, but since the cmdline in the PSP
> is restricted to 126 bytes, MS has extended this in MS-DOS 7.x : there's
> supposed to exist an environment variable CMDLINE, which may hold a cmdline
> of "any" size. HX's Win32 kernel emulation dkrnl32 uses this environment
> variable if possible.
FWIW, the details of the command-line tail are as follows.

There are 128 bytes set aside in the PSP for the command-tail. The first byte (at PSP:80h]) is the size (number of bytes) in the command-tail, and the last byte of the command-tail is always an ASCII 13 (Carriage Return or CR). That's why the command-tail size is limited to 126 bytes. In my programs (usually written in ASM), I normally ignore the size and just look for the CR.

The way the CMDLINE environment variable works is that if the command-tail is more than 126 bytes, the PSP still contains the first part of the tail and the size is shown as maximum (7Eh or 126). There is no direct indication anywhere that what's in the PSP is incomplete.

What I do in my programs is check the size byte (at PSP:[80h]) and if it's maxed out (7Eh) I look for a CMDLINE environment variable. If CMDLINE exists, I use it. If not, I revert back to what's in the PSP.

Others have adopted what MS did with Windows (e.g., when working at an NT command prompt there is a CMDLINE variable) so there is no way to test ahead of time whether it's supported or not. Also, a user can set an environment variable called CMDLINE to whatever they want so the mere presence of a CMDLINE variable doesn't guarantee anything about the command-tail.

Also, some environments/programs may assume that the command-tail is never more than 126 bytes, so may not properly be able to process a bigger one (you'll try to process a part of memory that isn't actually part of the command-tail)

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