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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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