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From: tcamp@ecsvax.UUCP (Ted A. Campbell)
Newsgroups: comp.sys.ibm.pc
Subject: Scrambled DOS-to-BIOS Function Calls (a query)
Message-ID: <3455@ecsvax.UUCP>
Date: Fri, 26-Jun-87 08:30:45 EDT
Article-I.D.: ecsvax.3455
Posted: Fri Jun 26 08:30:45 1987
Date-Received: Sat, 27-Jun-87 09:54:16 EDT
Organization: UNC Educational Computing Service
Lines: 22


I have written an interrupt handler that replaces interrrupt type 
0x10--the video display BIOS interrupt.  This interrupt handler
works well with direct calls to it, and with some DOS calls that
call it indirectly.

However, sometimes the function call in the AH register is odd when
it comes from DOS.  For example, if I do a DOS interrupt call 
(type 0x21) with function 2 to display an asterisk (ASCII code
0x2a), the DOS interrupt calls the video display interrupt (0x10)
with the correct value for the character in AL (0x2a), but with 
the function call for the BIOS interrupt set to 0xf1 (!), when 
it should be 0x9, 0xa, or (more likely) 0xe.  

Anyone have an idea as to why AH is getting scrambled between the
DOS interrupt and the video interrupt it calls?  Perhaps I've not
understood the process thoroughly.

tcamp@ecsvax
Ted A. Campbell
Duke University
Durham, NC