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t/data/zx48.asm  view on Meta::CPAN

                                ; which is to be discarded.

        JR      NC,L05BD        ; forward to LD-VERIFY if not to be loaded.

        LD      (IX+$00),L      ; place loaded byte at memory location.
        JR      L05C2           ; forward to LD-NEXT

; ---

;; LD-FLAG
L05B3:  RL      C               ; preserve carry (verify) flag in long-term
                                ; state byte. Bit 7 can be lost.

        XOR     L               ; compare type in A with first byte in L.
        RET     NZ              ; return if no match e.g. CODE vs. DATA.

;   continue when data type matches.

        LD      A,C             ; fetch byte with stored carry
        RRA                     ; rotate it to carry flag again
        LD      C,A             ; restore long-term port state.

        INC     DE              ; increment length ??
        JR      L05C4           ; forward to LD-DEC.
                                ; but why not to location after ?

; ---
;   for verification the byte read from tape is compared with that in memory.

;; LD-VERIFY
L05BD:  LD      A,(IX+$00)      ; fetch byte from memory.
        XOR     L               ; compare with that on tape
        RET     NZ              ; return if not zero. 

;; LD-NEXT
L05C2:  INC     IX              ; increment byte pointer.

;; LD-DEC
L05C4:  DEC     DE              ; decrement length.
        EX      AF,AF'          ; store the flags.
        LD      B,$B2           ; timing.

;   when starting to read 8 bits the receiving byte is marked with bit at right.
;   when this is rotated out again then 8 bits have been read.

;; LD-MARKER
L05C8:  LD      L,$01           ; initialize as %00000001

;; LD-8-BITS
L05CA:  CALL    L05E3           ; routine LD-EDGE-2 increments B relative to
                                ; gap between 2 edges.
        RET     NC              ; return with time-out.

        LD      A,$CB           ; the comparison byte.
        CP      B               ; compare to incremented value of B.
                                ; if B is higher then bit on tape was set.
                                ; if <= then bit on tape is reset. 

        RL      L               ; rotate the carry bit into L.

        LD      B,$B0           ; reset the B timer byte.
        JP      NC,L05CA        ; JUMP back to LD-8-BITS

;   when carry set then marker bit has been passed out and byte is complete.

        LD      A,H             ; fetch the running parity byte.
        XOR     L               ; include the new byte.
        LD      H,A             ; and store back in parity register.

        LD      A,D             ; check length of
        OR      E               ; expected bytes.
        JR      NZ,L05A9        ; back to LD-LOOP 
                                ; while there are more.

;   when all bytes loaded then parity byte should be zero.

        LD      A,H             ; fetch parity byte.
        CP      $01             ; set carry if zero.
        RET                     ; return
                                ; in no carry then error as checksum disagrees.

; -------------------------
; Check signal being loaded
; -------------------------
;   An edge is a transition from one mic state to another.
;   More specifically a change in bit 6 of value input from port $FE.
;   Graphically it is a change of border colour, say, blue to yellow.
;   The first entry point looks for two adjacent edges. The second entry point
;   is used to find a single edge.
;   The B register holds a count, up to 256, within which the edge (or edges) 
;   must be found. The gap between two edges will be more for a '1' than a '0'
;   so the value of B denotes the state of the bit (two edges) read from tape.

; ->

;; LD-EDGE-2
L05E3:  CALL    L05E7           ; call routine LD-EDGE-1 below.
        RET     NC              ; return if space pressed or time-out.
                                ; else continue and look for another adjacent 
                                ; edge which together represent a bit on the 
                                ; tape.

; -> 
;   this entry point is used to find a single edge from above but also 
;   when detecting a read-in signal on the tape.

;; LD-EDGE-1
L05E7:  LD      A,$16           ; a delay value of twenty two.

;; LD-DELAY
L05E9:  DEC     A               ; decrement counter
        JR      NZ,L05E9        ; loop back to LD-DELAY 22 times.

        AND      A              ; clear carry.

;; LD-SAMPLE
L05ED:  INC     B               ; increment the time-out counter.
        RET     Z               ; return with failure when $FF passed.

        LD      A,$7F           ; prepare to read keyboard and EAR port
        IN      A,($FE)         ; row $7FFE. bit 6 is EAR, bit 0 is SPACE key.