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t/data/zx48_base.asm  view on Meta::CPAN

;; S-SCRN$-S
S_SCRN__S:
        call STK_TO_BC          ; routine STK-TO-BC.
        ld hl,(0x5C36)          ; fetch address of CHARS.
        ld de,0x0100            ; fetch offset to chr$ 32
        add hl,de               ; and find start of bitmaps.
                                ; Note. not inc h. ??
        ld a,c                  ; transfer line to A.
        rrca                    ; multiply
        rrca                    ; by
        rrca                    ; thirty-two.
        and 0xE0                ; and with 11100000
        xor b                   ; combine with column $00 - $1F
        ld e,a                  ; to give the low byte of top line
        ld a,c                  ; column to A range 00000000 to 00011111
        and 0x18                ; and with 00011000
        xor 0x40                ; xor with 01000000 (high byte screen start)
        ld d,a                  ; register DE now holds start address of cell.
        ld b,0x60               ; there are 96 characters in ASCII set.

;; S-SCRN-LP
S_SCRN_LP:
        push bc                 ; save count
        push de                 ; save screen start address
        push hl                 ; save bitmap start
        ld a,(de)               ; first byte of screen to A
        xor (hl)                ; xor with corresponding character byte
        jr z,S_SC_MTCH          ; forward to S-SC-MTCH if they match
                                ; if inverse result would be $FF
                                ; if any other then mismatch

        inc a                   ; set to $00 if inverse
        jr nz,S_SCR_NXT         ; forward to S-SCR-NXT if a mismatch

        dec a                   ; restore $FF

; a match has been found so seven more to test.

;; S-SC-MTCH
S_SC_MTCH:
        ld c,a                  ; load C with inverse mask $00 or $FF
        ld b,0x07               ; count seven more bytes

;; S-SC-ROWS
S_SC_ROWS:
        inc d                   ; increment screen address.
        inc hl                  ; increment bitmap address.
        ld a,(de)               ; byte to A
        xor (hl)                ; will give $00 or $FF (inverse)
        xor c                   ; xor with inverse mask
        jr nz,S_SCR_NXT         ; forward to S-SCR-NXT if no match.

        djnz S_SC_ROWS          ; back to S-SC-ROWS until all eight matched.

; continue if a match of all eight bytes was found

        pop bc                  ; discard the
        pop bc                  ; saved
        pop bc                  ; pointers
        ld a,0x80               ; the endpoint of character set
        sub b                   ; subtract the counter
                                ; to give the code 32-127
        ld bc,0x0001            ; make one space in workspace.

        rst 0x30                ; BC-SPACES creates the space sliding
                                ; the calculator stack upwards.
        ld (de),a               ; start is addressed by DE, so insert code
        jr S_SCR_STO            ; forward to S-SCR-STO

; ---

; the jump was here if no match and more bitmaps to test.

;; S-SCR-NXT
S_SCR_NXT:
        pop hl                  ; restore the last bitmap start
        ld de,0x0008            ; and prepare to add 8.
        add hl,de               ; now addresses next character bitmap.
        pop de                  ; restore screen address
        pop bc                  ; and character counter in B
        djnz S_SCRN_LP          ; back to S-SCRN-LP if more characters.

        ld c,b                  ; B is now zero, so BC now zero.

;; S-SCR-STO
S_SCR_STO:
        jp STK_STO__            ; to STK-STO-$ to store the string in
                                ; workspace or a string with zero length.
                                ; (value of DE doesn't matter in last case)

; Note. this exit seems correct but the general-purpose routine S-STRING
; that calls this one will also stack any of its string results so this
; leads to a double storing of the result in this case.
; The instruction at L257D should just be a RET.
; credit Stephen Kelly and others, 1982.

; -------------
; Scanning ATTR
; -------------
; This function subroutine returns the attributes of a screen location -
; a numeric result.
; Again it's up to the BASIC programmer to supply valid values of line/column.

;; S-ATTR-S
S_ATTR_S:
        call STK_TO_BC          ; routine STK-TO-BC fetches line to C,
                                ; and column to B.
        ld a,c                  ; line to A $00 - $17   (max 00010111)
        rrca                    ; rotate
        rrca                    ; bits
        rrca                    ; left.
        ld c,a                  ; store in C as an intermediate value.

        and 0xE0                ; pick up bits 11100000 ( was 00011100 )
        xor b                   ; combine with column $00 - $1F
        ld l,a                  ; low byte now correct.

        ld a,c                  ; bring back intermediate result from C
        and 0x03                ; mask to give correct third of
                                ; screen $00 - $02