READ - Read sector from disk

Syntax:

CALL 4035H
Input:

  • The drive must be powered on.

  • Address of the sector to be read in SCTR (IY+17) and TRCK (IY+18) of the DOS vectors.

  • RETRY (IY+19) = number of read attempts

Exit:

The read sector is in the data buffer.

Registers used:

AF, BC, DE, HL

Error handling:

  • A = 0 Ok, sector was read

  • A = 9 An address mark was not found

  • A =10 Checksum wrong

  • A =17 BREAK key pressed

The sector addressed with SCTR and TRCK is transferred from the diskette to the data buffer.

The checksum is determined and compared at the end of the sector with the value stored there.

In the RETRY (IY+19) field of the DOS vectors you can specify how many read attempts should be made if the checksum is incorrect (default value = 10).

Example:

...
DI                  ; disable interrupts
LD (IY+11),80H      ; select Drive 2
CALL 4008H          ; and turn on
LD BC,50            ; 50 ms delay
CALL 4038H
LD (IY+17),14       ; set Sector Number in SCTR
LD (IY+18),28       ; set Track Number in TRCK
CALL 4035H          ; read sector
OR A                ; error occurred?
JP NZ,400EH         ; yes, to the ERROR routine
CALL 400BH          ; turn off drive
EI                  ; enable interrupts again
...

Sector 14 data on track 28 is transferred from the disk in drive 2 to the data buffer.

Internally called routines: IDAM