elwell@cwruecmp.UUCP (Clayton Elwell) (03/30/84)
In the process of disassembling and commenting PC-DOS 2.00
(no flames, please; I know it's masochistic but I am
insatiably curious), I have discovered the purpose of reserved
interrupt number 29H. It is used by the console driver to
output a character. This means that if you want a new console output
driver, all you need to do is intercept INT 29H and DOS will handle
mutli-byte transfers, device strategy and interrupts, etc. The
character to output is passed in the AL register. The default handler
does not seem to preserve the registers, but it might be a good idea to
do it just in case. keyboard handling remains the same.
This should allow much smaller screen drivers, since DOS takes care of
all of the overhead. Note: Since this is undocumented, it may well go
away in future versions (though it's still there in DOS 2.10). I don't
know if this "feature" is in generic MS-DOS 2.x.
Happy HAcking,
Clayton Elwell
elwell@cwruecmp.UUCP
...!decvax!cwruecmp!elwell
elwell%case@rand-relay
elwell@case.csnet
Clayton Elwell
...{usenet}!decvax!cwruecmp!elwell
ARPA: Elwell%Case@Rand-Relay
CSnet: Elwell@Case