NEC CP/M-86 - Page 121

BIOS Disk Definition Tables
Given n disk drives, the DPHs are arranged in a table whose first row of 16 bytes
corresponds to Drive 0, and whose last row corresponds to drive n-1. The table
appears as follows.
DPBASE
00 XLTOO
0000 0000 0000
DIRBUF DBPOO CSVOO ALVOO
01 XLT01
0000
0000
0000 DIRBUF DBP 01
CSV01 ALV01
(and so forth through)
The label DPBASE defines the offset of the DPH table relative to the beginning of
the operating system.
A responsibility
of the SELDSK subroutine, defined in Chapter 5, is to return the
offset
of the DPH from the beginning of the operating system for the selected drive.
The following sequence
of operations returns the table offset, with a value ofOOOOH
returned if the selected drive does not exist.
NDISKS EQU
8
;NUMBER OF DISK DRIVES
SELDSK:
;SELECT DISK N GIVEN BY CL
MOV BX,OOOOH ;READY FOR ERR
CMP CL,NDISKS ;N BEYOND MAX DISKS?
JNB RETURN ;RETURN IF SO
;0 <= N < NDISKS
MOV CH,O ;DOUBLE (N)
MOV BX,CX ;BX = N
MOV CL,4 ;READY FOR * 16
SHL BX,CL ;N = N * 16
MOV CX,OFFSET DPBASE
ADD BX,CX ;DPBASE + N * 16
RETURN: RET ;BX - .DPH (N)
7-3