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From: dan@srs.UUCP (Dan Kegel)
Newsgroups: comp.sys.ibm.pc
Subject: Re: BIOS question
Message-ID: <279@srs.UUCP>
Date: Wed, 9-Sep-87 16:14:31 EDT
Article-I.D.: srs.279
Posted: Wed Sep  9 16:14:31 1987
Date-Received: Fri, 11-Sep-87 06:44:07 EDT
References: <2265@sphinx.uchicago.edu>
Organization: S.R.Systems
Lines: 29

> I'm curious if the byte at F000:FFFF is meaningful in the PC BIOS. I
> notice that on some BIOS'es dated 10/27/82 this byte is "w" while on
> others it's "x."  Does this byte signify minor changes to the BIOS?

According to the IBM PS/2 & PC BIOS interface Tech Ref, only ffff:000e is 
defined, and it indicates the model, and only sometimes the BIOS version.
But who knows?

For newer BIOSes, though (XT after 1/10/86, PC Conv, XT 286, PS/2),
you can use the Get System Configuration call to get more info.  The call
is AH = C0h, INT 15h; on return, CF = 1 if BIOS doesn't support this call
(i.e. is early XT, etc), or sets ES:BX to point to a ROM table containing
the structure
    byte_count	dw	?	; number of bytes following
    model	db	?	; PC=ff, XT=fe or fb, PCjr = fd, etc, etc
    submodel	db	?	; distingushes between AT and XT/286, etc.
    BIOS_rev	db	?	; 0 for first release, 1 for 2nd, etc.
    featbyte	db	?	; 80h = DMA channel 3 used by hard disk BIOS,
				; 40h = 2nd 8259 installed,
				; 20h = RTC installed,
				; 10h = INT 15h called upon INT 9h
				;  8h = wait for external event supported,
				;  4h = extended BIOS area (at 640K) allocated,
				;  2h = bus is Micro Channel instead of PC.
				;  1h   reserved

(This manual is very useful for BIOS users- you can get it at 1-800-IBM-PCTB.)

- Dan Kegel