Commodore 1541 - Page 56

10. ASSEMBLY LANGUAGE AND THE 1541
If you want to use your 1541 disk drive to manipulate data directly from assembly
language you can use the information presented below.
Here is a list of subroutines that provide the start of memory locations in each of
the Kernal routines. These routines are used in conjunction with the assembly language
command JSR 10 jump to thai subroutine location in memory:
SUBROUTINE
SETLFS = SFFBA ; set logical, physical &. secondary addresses
SETNAM = SFFBD ; save length & address of filename
OPEN = SFFCO ; open a logical tile
CLOSK = SFFC3 ; close a logical file
CHKIN = SFFC6 ; open a channel far input
CT.RCII - SFFCC ; dear all channels
BASIN = SFFCF ; get a byte from a file
BSOUT = SFFD2 ; output a character to the screen
For a more complete description as to what each routine does and what
parameters are passed lo and from cadi routine, see your Commodore 64 or V1C-20
Programmer's Reference Guide.
Now, for a practical application of the subroutines listed above, here is a sample
program using (hose routines to read a sequential file on a disk. Assume that you have
stored the filename "TEST" at SC0O0.
INIT ; initialise:
LDA #$04 ; filename length
LDX #$00 ; low byte of filename address
LDV #$C0 ; high byte of filename address
JSR SETNAM ; save length & address of filename
LDA #503 ; logical address
LDX #$08 ; device number
LDY #500 ; secondary address (0 = read seq. file)
JSR SETLFS ; set logical, physical & secondary addresses
JSR OPEN ; open logical file
LDX #$03 ; sei x-rcgislcr to logical address
JSR CHKIN ; open input channel
your program info ; get data and print it one byte at a time
JSR BASIN ; get one byte
BEQ END ; if 0 then end of file or error
JSR BSOUT ; output character to the screen
JMP jour prog, info ; loop
END
LDA #503 ; set accumulator to logical address
JSR CLOSE ; close Hie
JSR CLRCH ; clear channels and reset defaults
RTS ; end of assembly language program
SO