Chapter 4
Basic Disk Operating System
(BDOS) Functions
This chapter presents the interface conventions which allow transient
program
access to
CP
1M
-86 BOOS functions. The BOOS calls correspond closely
to
CP
IM-
80
Version 2 in order
to
simplify translation
of
existing CP/M-80 programs for
operation under
CP
IM-86.
BOOS entry
and
exit conditions are described first,
followed by the individual
BOOS function calls.
BDOS
PARAMETERS
Entry to the BOOS
is
accomplished through the 8086 software interrupt #224,
which
is
reserved by Intel Corporation for use by CP/M-86. The function code
is
passed in register CL, with byte parameters in
OL
and
word parameters in ox.
Single byte values are returned in AL, word values in both
AX
and
BX,
and
double-word values in ES
and
BX. All segment registers, except ES, are saved
upon
entry
and
restored
upon
exit from the BOOS (corresponding to
PL/M-86
conven-
tions). Table
4-1
summarizes input
and
output
parameter passing.
Table 4-1
BDOS
Parameter
Summary
BOOS ENTRY REGISTERS
BOOS RETURN REGISTERS
CL
Function Code Byte value returned in AL
DL
Byte Parameter
Word value returned
in
both AX
and
BX
DX
Word
Parameter
Double-word value returned with
DS
Data
Segment offset in
BX
and
segment in
ES
4-1
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