IDAM - Look for the address mark on the diskette

Syntax:

CALL 4023H
Input:

  • The corresponding drive must be switched on.

  • SCTR (IY+17) = Sector Number

  • TRCK (IY+18) = Track Number

Exit:

If the return jump is error-free, the read/write head is located directly in front of the data mark of the sector being searched for.

Registers used:

AF, BC. DE, HL

Error handling:

  • A = 0 - Address mark found

  • A = 9 - Address mark not found

  • A = 17 - BREAK key pressed

  • if A = 0, Z flag is set

  • if A <> 0, Z flag is clear

This routine is used to position the read/write head in front of the data mark of this sector before writing or reading a sector.

IDAM first positions the head over the desired track and then reads address mark after address mark until the desired sector is found. The writing or reading process for the data field must then begin immediately, since the diskette continues to rotate.

IDAM is already integrated in the READ and WRITE routines for reading or writing a sector.

Example:

...
DI              ; disable interrupts
LD A,80H        ; select Drive 2
LD (IY+11),A
CALL 4008H      ; and turn on
LD BC,50        ; 50 ms delay
CALL 4038H
LD (IY+17),6    ; Sector Number
LD (IY+18),14   ; Track Number
CALL 4023       ; search sector
JP NZ,400EH     ; error or BREAK
...
...     Read or write sector
...

The read/write head should be positioned in front of the data mark of sector 6 of track 14 for subsequent reading or writing.

Internally called routines: STPIN, STPOUT