M.J.Karas, 00-00-00
DETERMINE LOGGED DISK: Function 25.
An applications program can determine which disk drive is
the currently logged or default drive through use of this
function. The BDOS will return in the (A) register the number of
the currently selected drive according to the table given above.
The program segment below shows a sequence of BDOS interface
code that first determines if drive B: is selected, and if not
then does a BDOS call to change it.
;SELECT AND POLL LOGGED DISK DRIVE EXAMPLE
;
SELECT EQU 0EH ; FUNCTION 14
ASKDRV EQU 19H ; FUNCTION 25
BDOS EQU 0005H ; SYSTEM ENTRY POINT
ORG 0100H ; PROG START
LD C,ASKDRV ; FIND OUT IF B: IS SELECTED
CALL BDOS
CP 'B'-'A'
RET Z ; DONT SELECT IF ALREADY
; ..LOGGED
LD E,'B'-'A' ; SET TO LOG AND SELECT B:
LD C,SELECT
CALL BDOS
RET ; FINISHED WITH ANOTHER PROG
;
END
DRIVE STATUS SET AND RESET: Functions 28 & 37.
Drive status may be individually controlled by these
functions. Operation 28 allows a the currently selected drive to
be write protected (set to read/only). The process is simply:
WPDSK EQU 01CH
BDOS EQU 0005H
LD C,WPDSK ;WRITE PROTECT DISK
CALL BDOS
The write protect status of a specific disk may be removed by
function 37 which deactivates the directories of each drive
specified at call time. Each drive by default then becomes
read/write again but requires reactivation through reselection.
The reset drive vector is a 16-bit value passed to the BDOS with
a "1" bit in each bit position for a drive that equires
resetting. The most significant bit of the 16 bit quanity
corresponds to drive P: and the LSB to drive A:. The code
sequence to reset drive B: would be:
RESDSK EQU 025H
BDOS EQU 0005H
LD C,RESDSK ;FUNCTION CODE
LD DE,0000$0000$0000$0010B ;DRIVE B: BIT SET
CALL BDOS
GET DRIVE LOGIN AND READ?ONLY VECTORS: Function 24 & 29.
The BDOS keeps track of all drives that have been selected
since the last boot or disk reset functions. These drives are
considered in a online status in that the system knows
immediately what the space allocation map of the drive is and
whether the drive is in read/only status or not. Function 24
allows the application program to determine what subset of the
current drive complement are in this online logged status. The
vector returned in the (HL) register pair is a bit map like above
where a "1" bit means the drive is active. The most significant
bit of the 16-bit number corresponds to drive P:. The code below
fetches the vector and saves it in a local data area.
;LOGIN VECTOR EXAMPLE
;
LOGIN EQU 018H ; FUNCTION 24
BDOS EQU 0005H ; SYSTEM ENTRY POINT
ORG 0100H
LD C,LOGIN ; FUNCTION
CALL BDOS
LD (LOCLOG),HL ; SAVE VECTOR HERE
RET ; TO CCP
;
LOCLOG:
DEFS 2
END
In a similar manner the BDOS allows determination of which
drives are in the write protected read/only status. A "1" bit in
the returned vector indicates read/only status for a specific
drive. The code here shows how to fetch it.
;READ/ONLY VECTOR EXAMPLE
;
ROVEC EQU 01DH ; FUNCTION 29
BDOS EQU 0005H ; SYSTEM ENTRY POINT
ORG 0100H
LD C,ROVEC ; FUNCTION
CALL BDOS
LD (LOCROV),HL ; SAVE VECTOR HERE
RET ; TO CCP
;
LOCROV:
DEFS 2
END
GET ALLOCATION VECTOR AND DISK PARM POINTER: Function 27 & 31.
Two more miscellaneous disk drive interface functions are
provided that permit several special types of functions to be
performed. The first, function 27 returns an address in the (HL)
registers that points to a bit string in memory that corresponds
to the data block allocation map of the currently selected drive.
The map contains one bits in each position where a block
allocated, starting with the MSB of the forst byte in the string.
The length of the bit string depends upon the total capacity of
the drive in allocatable blocks. Function 31 permits an
application to determine the characteristics of the currently
selected drive. The BDOS returns an address in the (HL) registers
that points to a table of 33 bytes that describe the current
drive. Data in the table includes such data as number of
possible directory entries on the disk, number of allocatable
blocks on the disk, and, indirectly, the size of each disk block.
The program below is a comprehensive example of how these
functions can be used to determine the remaining space left on a
the selected drive. The program stores the available space of the
drive specified in the first byte of the default FCB into memory
location "KPDISK" and then exits to the CCP. The reader can adapt
the code as desired.
;
;CP/M BDOS INTERFACE EQUATES
;
BASE EQU 0000H ;BASE OF CP/M SYSTEM
LOGDRIV EQU 0004H+BASE ;LOCATION OF CURRENTLY LOGGED DRIVE
BDOS EQU 0005H+BASE ;THE BDOS I/O VECTOR
SLCTDSK EQU 14 ;SELECT DISK DRIVE
GALVEC EQU 27 ;GET ADDRESS ALLOCATION VECTOR
GDSKP EQU 31 ;GET ADDRESS OF DISK PARAMETER TABLE
;
;
ORG 0100H
;
;
;PROGRAM TO FETCH REMAINING DISK SPACE IN KBYTES
;
SPCGET:
LD A,(LOGDRIV) ; GET CURRENTLY LOGGED DRIVE AND SAVE
AND 0FH ; STRIP OUT USER NUMBER
LD (SAVDRIV),A ; SAVE CODE
;
LD A,(FCB) ; CHECK IF SAME AS SELECT
DEC A ; ADJUST FCB DRIVE TO MATCH SELECT DRIVE
LD E,A ; ..SELECT IN BDOS
LD C,SLCTDSK ; SELECT DISK FUNCTION
CALL BDOS
;
LD C,GDSKP ; FIND ADDRESS OF DISK PARAMETER HEADER
CALL BDOS
LD BC,0002H ; INDEX TO BLOCK SHIFT FACTOR
ADD HL,BC
LD B,(HL) ; (B) = BYTE BLOCK SHIFT FACTOR
INC HL
INC HL
INC HL
LD E,(HL) ; (DE) = WORD DISK BLOCK COUNT
INC HL
LD D,(HL)
INC DE
;
LD A,B ; ADJUST SHIFT FOR KBYTE SIZE
SUB 03H
LD HL,0001H ; CALCULATE BLOCK SIZE
SPCCAL:
OR A ; KNOW KBYTES PER BLOCK?
JP Z,SPCKNW
ADD HL,HL ; DOUBLE # SECTORS PER TRACK
DEC A ; DECREMENT BLOCK SHIFT
JP SPCCAL
;
SPCKNW:
LD C,L ; (BC)=KBYTES PER BLOCK
LD B,H
LD HL,0 ; INITIALIZE KPDISK
LD (KPDISK),HL
PUSH BC ; SAVE KBYTES/BLOCK
PUSH DE ; SAVE NUMBER OF BLOCKS
LD C,GALVEC ; NOW POINT TO THE ALLOCATION VECTOR
CALL BDOS ; (HL)=ALLOCATION VECTOR ADDRESS
POP DE
POP BC
;
LD (ALLSAVE),HL ; SAVE ALLOCATION POINTER
LD H,1 ; SET MINIMUM START BIT COUNT
;
UALLOC:
DEC H ; DEC BIT COUNT
JP NZ,STACT ; STILL ACTIVE BYTE
;
LD HL,(ALLSAVE) ; GET POINTER
LD A,(HL)
INC HL
LD (ALLSAVE),HL ; SAVE NEW POINTER
LD H,08H ; SET BIT COUNTER TO MAX
;
STACT:
RLCA ; GET ALLOCATION BIT TO CARRY
JP C,ALLOC ; DONT COUNT ALLOCATED BLOCKS
PUSH HL
LD HL,(KPDISK) ; GET KBYTES LEFT COUNT
ADD HL,BC ; ADD IN ONE MORE BLOCK COUNT
LD (KPDISK),HL
POP HL
;
ALLOC:
DEC DE ; DEC TOTAL BLOCK COUNT
LD L,A
LD A,D
OR E ; ALL BLOCKS SCANNED YET
LD A,L ; RESTORE ALLOC BIT PATTERN
JP NZ,UALLOC ; MORE TO COUNT
;
LD A,(SAVDRIV) ; RETURN DISK SELECT TO PREVIOUS
LD E,A ; ..SELECT IN BDOS
LD C,SLCTDSK ; SELECT DISK FUNCTION
CALL BDOS
RET ; BACK TO THE CCP
;
;
;PROGRAM DATA STORAGE ALLOCATIONS
;
BLKSIZ:
DEFS 2 ; STORAGE FOR ALLOCATION BLOCK SIZE
ALLSAVE:
DEFS 2 ; STORAGE FOR ALLOCATION PNT SAVE
SAVDRIV:
DEFS 1 ; SAVE CURRENT DISK SELECT DURING RELOG
KPDISK:
DEFS 2 ; STORAGE FOR KBYTES PER DRIVE LEFT
;
END
The next part in this series will present the the CP/M file
system as viewed from the BDOS interface aspect. The FILE CONTROL
BLOCK (FCB) will be presented. In addition the procedures to
prepare files for I/O and then the actual I/O procedures will be
presented. The series will round out to a conclusion with a
comprehensive programming example that presents a sequential file
I/O set of subroutines that permit character by character I/O
with a file to be done.
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