![]() The main thing it did was self-diagnostics to determine if the system had been infected by a boot sector virus. I wrote my own DOS clone in Assembler, that I called PoverDOS because it was surely going to make me about that much money, and managed to make a boot sector that was far more functional than the one that comes standard with MS and PC DOS. The first bytes of the PSP are a literal INT 20h which is the Drop To DOS interrupt, and a RET with nothing left on the stack will return you to that zero offset, and then Drop to DOS. It is why you could simply execute a RET (return) and have it drop back to DOS again. ![]() COM file's entry point, called the Program Segment Prefix (PSP). One thing that I always thought was interesting was all the stuff that gets packed in the first 100h bytes before a. I studied MS-DOS inside and out (quite literally).
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