+ cpi temp,WRUAL ;write unallocated?
+ brne dsk_chkuna ;check for unalloc
+
+; write to unallocated, set parameters
+ ldi temp,blksize/128 ;next unalloc recs
+ sts unacnt,temp
+ lds temp,seekdsk ;disk to seek
+ sts unadsk,temp ;unadsk = sekdsk
+ lds temp,seektrk
+ sts unatrk,temp ;unatrk = sectrk
+ lds temp,seektrk+1
+ sts unatrk+1,temp ;unatrk = sectrk
+ lds temp,seeksec
+ sts unasec,temp ;unasec = seksec
+;
+dsk_chkuna:
+ ;check for write to unallocated sector
+ lds temp,unacnt ;any unalloc remain?
+ tst temp
+ breq dsk_alloc ;skip if not
+
+; more unallocated records remain
+ dec temp ;unacnt = unacnt-1
+ sts unacnt,temp
+ lds temp,seekdsk ;same disk?
+ lds temp2,unadsk
+ cp temp,temp2 ;seekdsk = unadsk?
+ brne dsk_alloc ;skip if not
+
+; disks are the same
+ lds temp,unatrk
+ lds temp2,unatrk+1
+ lds temp3,seektrk
+ lds temp4,seektrk+1
+ cp temp,temp3 ;seektrk = unatrk?
+ cpc temp2,temp4
+ brne dsk_alloc ;skip if not
+
+; tracks are the same
+ lds temp,seeksec ;same sector?
+ lds temp2,unasec
+ cp temp,temp2 ;seeksec = unasec?
+ brne dsk_alloc ;skip if not
+
+; match, move to next sector for future ref
+ inc temp2 ;unasec = unasec+1
+ sts unasec,temp2
+ cpi temp2,CPMSPT ;end of track? (count CP/M sectors)
+ brlo dsk_noovf ;skip if no overflow
+
+; overflow to next track
+ sts unasec,_0 ;unasec = 0
+ lds temp,unatrk
+ lds temp2,unatrk+1
+ subi temp, low(-1) ;unatrk = unatrk+1
+ sbci temp2,high(-1)
+ sts unatrk,temp
+ sts unatrk+1,temp2
+;
+dsk_noovf:
+ cbi flags,rsflag ;rsflag = 0
+ rjmp dsk_rwoper ;to perform the write
+;
+dsk_alloc:
+ ;not an unallocated record, requires pre-read
+ sts unacnt,_0 ;unacnt = 0
+ sbi flags,rsflag ;rsflag = 1
+
+;*****************************************************
+;* Common code for READ and WRITE follows *
+;*****************************************************
+
+dsk_rwoper:
+ ;enter here to perform the read/write
+ sts erflag,_0 ;no errors (yet)
+
+ ;Convert track/sector to an LBA address (in 128byte blocks)
+
+ lds adrl,seeksec ;
+ ldi adrh,0 ;
+ lds temp3,seektrk ;
+ lds temp4,seektrk+1 ;
+ ldi temp,CPMSPT ;
+ mul temp3,temp ;
+ add adrl,r0 ;
+ adc adrh,r1 ;
+ mul temp4,temp ;
+ add adrh,r0 ;adrh:adrl := sec + trk * SectorsPerTrack
+ clr _0
+
+ mov temp,adrl
+ andi temp,SECMSK ;mask buffer number
+ push temp ;save for later
+.if DISK_DEBUG
+ rcall printstr
+ .db "; bufnr: ",0,0
+ rcall printhex
+.endif