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Basic I/O System (BIOS) Functions
THE BIOS JUMP VECTOR
Entry to the BIOS is through ajump vector located at offset 2500H from the base of
the operating system. The jump vector is a sequence of 31 three-byte jump instruc-
tions which transfer program control to the individual BIOS entry points. Although
some nonessential BIOS subroutines may contain a single return (RET) instruction,
the corresponding jump vector element must be present in the location shown in
Table 5-1.
Parameters for the individual subroutines in the BIOS are passed in the
ex
and DX
registers, when required.
ex
holds the first parameter; DX is used for a second
argument. Return values are passed in the registers according to type.
• Byte values are returned in AL.
• Word values ( 16 bits) are returned in BX.
Specific parameters and returned values are described with each subroutine.
BIOS SUBROUTINES
There are three major divisions in the BIOS jump table: system (re)initialization
subroutines, simple character I/O subroutines, and disk I/O subroutines. The
BIOS subroutines for disk I/O differ for floppy and hard disks. However, the same
jump vector is used for entry to the subroutine for both types of disk. The system
distinguishes between the media based on the value of the currently selected drive
specifier. Drives A through D identify floppy disk drives. Drives E and F corre-
spond to hard disk unit #0, while Drives G and H correspond to hard disk unit # 1.
The system supports a maximum of four floppy diskette drives and two hard disk
units (four logical hard disk drives).
5-3
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