Save and load: read

       Name: read                                                    [Show more]
       Type: Subroutine
   Category: Save and load
    Summary: Read a sector's worth of data into the buffr2 buffer
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * rttrk calls read


 This routine is identical to the READ16 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.


.read

 LDY #32                ; READ16   LDY #$20       ; 'MUST FIND' COUNT.

.read2

 DEY                    ; RSYNC    DEY IF         ; CAN'T FIND MARKS
 BEQ readE              ;          BEQ RDERR      ; THEN EXIT WITH CARRY SET.

.read3

 LDA Q6L,X              ; READ1    LDA Q6L,X      ; READ NIBL.
 BPL read3              ;          BPL READ1      ; *** NO PAGE CROSS! ***

.read4

 EOR #$D5               ; RSYNC1   EOR #$D5       ; DATA MARK 1?
 BNE read2              ;          BNE RSYNC      ; LOOP IF NOT.
 NOP                    ;          NOP DELAY      ; BETWEEN NIBLS.

.read5

 LDA Q6L,X              ; READ2    LDA Q6L,X
 BPL read5              ;          BPL READ2      ; *** NO PAGE CROSS! ***
 CMP #$AA               ;          CMP #$AA       ; DATA MARK 2?
 BNE read4              ;          BNE RSYNC1     ; (IF NOT, IS IT DM1?)
 LDY #$56               ;          LDY #$56       ; INIT NBUF2 INDEX.
                        ; *       (ADDED NIBL DELAY)

.read6

 LDA Q6L,X              ; READ3    LDA Q6L,X
 BPL read6              ;          BPL READ3      ; *** NO PAGE CROSS! ***
 CMP #$AD               ;          CMP #$AD       ; DATA MARK 3?
 BNE read4              ;          BNE RSYNC1     ; (IF NOT, IS IT DM1?)
                        ; *       (CARRY SET IF DM3!)
 LDA #0                 ;          LDA #$00       ; INIT CHECKSUM.

.read7

 DEY                    ; RDATA1   DEY
 STY ztemp0             ;          STY IDX

.read8

 LDY Q6L,X              ; READ4    LDY Q6L,X
 BPL read8              ;          BPL READ4      ; *** NO PAGE CROSS! ***
 EOR rtable-$96,Y       ;          EOR DNIBL,Y    ; XOR 6-BIT NIBL.
 LDY ztemp0             ;          LDY IDX
 STA buffr2+256,Y       ;          STA NBUF2,Y    ; STORE IN NBUF2 PAGE.
 BNE read7              ;          BNE RDATA1     ; TAKEN IF Y-REG NONZERO.

.read9

 STY ztemp0             ; RDATA2   STY IDX

.readA

 LDY Q6L,X              ; READ5    LDY Q6L,X
 BPL readA              ;          BPL READ5      ; *** NO PAGE CROSS! ***
 EOR rtable-$96,Y       ;          EOR DNIBL,Y    ; XOR 6-BIT NIBL.
 LDY ztemp0             ;          LDY IDX
 STA buffr2,Y           ;          STA NBUF1,Y    ; STORE IN NBUF1 PAGE.
 INY                    ;          INY
 BNE read9              ;          BNE RDATA2

.readB

 LDY Q6L,X              ; READ6    LDY Q6L,X      ; READ 7-BIT CSUM NIBL.
 BPL readB              ;          BPL READ6      ; *** NO PAGE CROSS! ***
 CMP rtable-$96,Y       ;          CMP DNIBL,Y    ; IF LAST NBUF1 NIBL NOT
 BNE readE              ;          BNE RDERR      ; EQUAL CHKSUM NIBL THEN ERR.

.readC

 LDA Q6L,X              ; READ7    LDA Q6L,X
 BPL readC              ;          BPL READ7      ; *** NO PAGE CROSS! ***
 CMP #$DE               ;          CMP #$DE       ; FIRST BIT SLIP MARK?
 BNE readE              ;          BNE RDERR      ; (ERR IF NOT)
 NOP                    ;          NOP DELAY      ; BETWEEN NIBLS.

.readD

 LDA Q6L,X              ; READ8    LDA Q6L,X
 BPL readD              ;          BPL READ8      ; *** NO PAGE CROSS! ***
 CMP #$AA               ;          CMP #$AA       ; SECOND BIT SLIP MARK?
 BEQ readF              ;          BEQ RDEXIT     ; (DONE IF IT IS)

.readE

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

.readF

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