Save and load: rdaddr

       Name: rdaddr                                                  [Show more]
       Type: Subroutine
   Category: Save and load
    Summary: Read a track address field
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * trytrk calls rdaddr


 This routine is identical to the RDADR16 routine in Apple DOS 3.3.

 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.


.rdaddr

 LDY #$FC               ; RDADR16  LDY #$FC
 STY ztemp0             ;          STY COUNT      ; 'MUST FIND' COUNT.

.rdadr2

 INY                    ; RDASYN   INY
 BNE rdadr3             ;          BNE RDA1       ; LOW ORDER OF COUNT.
 INC ztemp0             ;          INC COUNT      ; (2K NIBLS TO FIND
 BEQ rdadrD             ;          BEQ RDERR      ; ADR MARK, ELSE ERR)

.rdadr3

 LDA Q6L,X              ; RDA1     LDA Q6L,X      ; READ NIBL.
 BPL rdadr3             ;          BPL RDA1       ; *** NO PAGE CROSS! ***

.rdadr4

 CMP #$D5               ; RDASN1   CMP #$D5       ; ADR MARK 1?
 BNE rdadr2             ;          BNE RDASYN     ; (LOOP IF NOT)
 NOP                    ;          NOP ADDED      ; NIBL DELAY.

.rdadr5

 LDA Q6L,X              ; RDA2     LDA Q6L,X
 BPL rdadr5             ;          BPL RDA2       ; *** NO PAGE CROSS! ***
 CMP #$AA               ;          CMP #$AA       ; ADR MARK 2?
 BNE rdadr4             ;          BNE RDASN1     ; (IF NOT, IS IT AM1?)
 LDY #3                 ;          LDY #$3        ; INDEX FOR 4-BYTE READ.
                        ; *       (ADDED NIBL DELAY)

.rdadr6

 LDA Q6L,X              ; RDA3     LDA Q6L,X
 BPL rdadr6             ;          BPL RDA3       ; *** NO PAGE CROSS! ***
 CMP #$96               ;          CMP #$96       ; ADR MARK 3?
 BNE rdadr4             ;          BNE RDASN1     ; (IF NOT, IS IT AM1?)
                        ; *       (LEAVES CARRY SET!)
 LDA #0                 ;          LDA #$0        ; INIT CHECKSUM.

.rdadr7

 STA ztemp1             ; RDAFLD   STA CSUM

.rdadr8

 LDA Q6L,X              ; RDA4     LDA Q6L,X      ; READ 'ODD BIT' NIBL.
 BPL rdadr8             ;          BPL RDA4       ; *** NO PAGE CROSS! ***
 ROL A                  ;          ROL A          ; ALIGN ODD BITS, '1' INTO
                        ;                           LSB.
 STA ztemp0             ;          STA LAST       ; (SAVE THEM)

.rdadr9

 LDA Q6L,X              ; RDA5     LDA Q6L,X      ; READ 'EVEN BIT' NIBL.
 BPL rdadr9             ;          BPL RDA5       ; *** NO PAGE CROSS! ***
 AND ztemp0             ;          AND LAST       ; MERGE ODD AND EVEN BITS.
 STA idfld,Y            ;          STA CSSTV,Y    ; STORE DATA BYTE.
 EOR ztemp1             ;          EOR CSUM       ; XOR CHECKSUM.
 DEY                    ;          DEY
 BPL rdadr7             ;          BPL RDAFLD     ; LOOP ON 4 DATA BYTES.
 TAY                    ;          TAY IF         ; FINAL CHECKSUM
 BNE rdadrD             ;          BNE RDERR      ; NONZERO, THEN ERROR.

.rdadrA

 LDA Q6L,X              ; RDA6     LDA Q6L,X      ; FIRST BIT-SLIP NIBL.
 BPL rdadrA             ;          BPL RDA6       ; *** NO PAGE CROSS! ***
 CMP #$DE               ;          CMP #$DE
 BNE rdadrD             ;          BNE RDERR      ; ERROR IF NONMATCH.
 NOP                    ;          NOP DELAY      ; BETWEEN NIBLS.

.rdadrB

 LDA Q6L,X              ; RDA7     LDA Q6L,X      ; SECOND BIT-SLIP NIBL.
 BPL rdadrB             ;          BPL RDA7       ; *** NO PAGE CROSS! ***
 CMP #$AA               ;          CMP #$AA
 BNE rdadrD             ;          BNE RDERR      ; ERROR IF NONMATCH.

.rdadrC

 CLC                    ; RDEXIT   CLC CLEAR      ; CARRY ON
 RTS                    ;          RTS NORMAL     ; READ EXITS.

.rdadrD

 SEC                    ; RDERR    SEC INDICATE   ; 'ERROR EXIT'.
 RTS                    ;          RTS RETURN     ; FROM READ16 OR RDADR16.