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System calls are realized by assembler command
int 40h
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Before the call eax should be set to the system function number.
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Many functions require parameters. Parameters are passed in other registers
(ebx,ecx,edx and so on).
The number and the type of parameters are explained in the documentation for each function.
- Many functions return result in the register eax,
and some set up other registers also. Presence/absence of returned value is explained in the documentation for each function.
- All registers except those with returned value preserve.
(Including flags register eflags).