Save and load: seek

       Name: seek                                                    [Show more]
       Type: Subroutine
   Category: Save and load
    Summary: Fast seek routine
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * rwts calls seek
             * trytrk calls seek


 This routine is almost identical to the SEEK routine in Apple DOS 3.3. There
 is one extra instruction and one moved instruction when compared to the
 original DOS.

 These extra instructions double the track number in A.

 For a detailed look at how DOS works, see the book "Beneath Apple DOS" by Don
 Worth and Pieter Lechner. In particular, see chapter 4 for the layout of the
 VTOC, catalog sector, file entry and file/track list.

 Elite uses different label names to the original DOS 3.3 source, but the code
 is the same.

 This code forms part of the RWTS ("read/write track sector") layer from Apple
 DOS, which was written by Randy Wigginton and Steve Wozniak. It implements the
 low-level functions to read and write Apple disks, and is included in Elite so
 the game can use the memory that's normally allocated to DOS for its own use.


 Arguments:

   A                    The track number


.seek

 STX ztemp0             ; SEEK     STX SLOTTEMP   ; SAVE X-REG
                        ;          STA TRKN       ; SAVE TARGET TRACK

 ASL A                  ; This is an extra instruction that doubles the track
                        ; number in A

 CMP curtrk             ;          CMP CURTRK     ; ON DESIRED TRACK?
 BEQ step3              ;          BEQ SEEKRTS    ; YES, RETURN

 STA ztemp1             ; This is the second instruction from above, which has
                        ; been moved here (STA TRKN)
                        ;
                        ; This saves the now-doubled track number in ztemp1

 LDA #0                 ;          LDA #$0
 STA ztemp2             ;          STA TRKCNT     ; HALFTRACK COUNT.

.seek2

 LDA curtrk             ; SEEK2    LDA CURTRK     ; SAVE CURTRK FOR
 STA ztemp3             ;          STA PRIOR      ; DELAYED TURNOFF.
 SEC                    ;          SEC
 SBC ztemp1             ;          SBC TRKN       ; DELTA-TRACKS.
 BEQ seek7              ;          BEQ SEEKEND    ; BR IF CURTRK=DESTINATION
 BCS seek3              ;          BCS OUT        ; (MOVE OUT, NOT IN)
 EOR #$FF               ;          EOR #$FF       ; CALC TRKS TO GO.
 INC curtrk             ;          INC CURTRK     ; INCR CURRENT TRACK (IN).
 BCC seek4              ;          BCC MINTST     ; (ALWAYS TAKEN)

.seek3

 ADC #$FE               ; OUT      ADC #$FE       ; CALC TRKS TO GO.
 DEC curtrk             ;          DEC CURTRK     ; DECR CURRENT TRACK (OUT).

.seek4

 CMP ztemp2             ; MINTST   CMP TRKCNT
 BCC seek5              ;          BCC MAXTST     ; AND 'TRKS MOVED'.
 LDA ztemp2             ;          LDA TRKCNT

.seek5

 CMP #12                ; MAXTST   CMP #$C
 BCS seek6              ;          BCS STEP2      ; IF TRKCNT>$B LEAVE Y ALONE
                        ;                           (Y=$B).
 TAY                    ; STEP     TAY            ; ELSE SET ACCELERATION INDEX
                        ;                           IN Y

.seek6                  ; STEP2    EQU *

 SEC                    ;          SEC            ; CARRY SET=PHASE ON
 JSR step               ;          JSR SETPHASE   ; PHASE ON
 LDA armtab,Y           ;          LDA ONTABLE,Y  ; FOR 'ONTIME'.
 JSR armwat             ;          JSR MSWAIT     ; (100 USEC INTERVALS)
                        ; *
 LDA ztemp3             ;          LDA PRIOR
 CLC                    ;          CLC            ; CARRY CLEAR=PHASE OFF
 JSR step2              ;          JSR CLRPHASE   ; PHASE OFF
 LDA armtb2,Y           ;          LDA OFFTABLE,Y ; THEN WAIT 'OFFTIME'.
 JSR armwat             ;          JSR MSWAIT     ; (100 USEC INTERVALS)
 INC ztemp2             ;          INC TRKCNT     ; 'TRACKS MOVED' COUNT.
 BNE seek2              ;          BNE SEEK2      ; (ALWAYS TAKEN)
                        ; *
.seek7                  ; SEEKEND  EQU *          ; END OF SEEKING

 JSR armwat             ;          JSR MSWAIT     ; A=0: WAIT 25 MS SETTLE
 CLC                    ;          CLC            ; AND TURN OFF PHASE
                        ; *
                        ; * TURN HEAD STEPPER PHASE ON/OFF
                        ; *

.step                   ; SETPHASE EQU *

 LDA curtrk             ;          LDA CURTRK     ; GET CURRENT PHASE

.step2                  ; CLRPHASE EQU *

 AND #3                 ;          AND #3         ; MASK FOR 1 OF 4 PHASES
 ROL A                  ;          ROL A          ; DOUBLE FOR PHASE INDEX
 ORA ztemp0             ;          ORA SLOTTEMP
 TAX                    ;          TAX
 LDA phsoff,X           ;          LDA PHASEOFF,X ; FLIP THE PHASE
 LDX ztemp0             ;          LDX SLOTTEMP   ; RESTORE X-REG

.step3

 RTS                    ; SEEKRTS  RTS            ; AND RETURN