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t/data/zx48.asm  view on Meta::CPAN

; Difference routine
; ------------------
; This routine terminates the above routine and is also called from the
; start of the next routine to calculate the length to reclaim.

;; DIFFER
L19DD:  AND     A               ; prepare for true subtraction.
        SBC     HL,DE           ; subtract the two pointers.
        LD      B,H             ; transfer result
        LD      C,L             ; to BC register pair.
        ADD     HL,DE           ; add back
        EX      DE,HL           ; and switch pointers
        RET                     ; return values are the length of area in BC,
                                ; low pointer (previous) in HL,
                                ; high pointer (next) in DE.

; -----------------------
; Handle reclaiming space
; -----------------------
;

;; RECLAIM-1
L19E5:  CALL    L19DD           ; routine DIFFER immediately above

;; RECLAIM-2
L19E8:  PUSH    BC              ;

        LD      A,B             ;
        CPL                     ;
        LD      B,A             ;
        LD      A,C             ;
        CPL                     ;
        LD      C,A             ;
        INC     BC              ;

        CALL    L1664           ; routine POINTERS
        EX      DE,HL           ;
        POP     HL              ;

        ADD     HL,DE           ;
        PUSH    DE              ;
        LDIR                    ; copy bytes

        POP     HL              ;
        RET                     ;

; ----------------------------------------
; Read line number of line in editing area
; ----------------------------------------
; This routine reads a line number in the editing area returning the number
; in the BC register or zero if no digits exist before commands.
; It is called from LINE-SCAN to check the syntax of the digits.
; It is called from MAIN-3 to extract the line number in preparation for
; inclusion of the line in the BASIC program area.
;
; Interestingly the calculator stack is moved from its normal place at the
; end of dynamic memory to an adequate area within the system variables area.
; This ensures that in a low memory situation, that valid line numbers can
; be extracted without raising an error and that memory can be reclaimed
; by deleting lines. If the stack was in its normal place then a situation
; arises whereby the Spectrum becomes locked with no means of reclaiming space.

;; E-LINE-NO
L19FB:  LD      HL,($5C59)      ; load HL from system variable E_LINE.

        DEC     HL              ; decrease so that NEXT_CHAR can be used
                                ; without skipping the first digit.

        LD      ($5C5D),HL      ; store in the system variable CH_ADD.

        RST     20H             ; NEXT-CHAR skips any noise and white-space
                                ; to point exactly at the first digit.

        LD      HL,$5C92        ; use MEM-0 as a temporary calculator stack
                                ; an overhead of three locations are needed.
        LD      ($5C65),HL      ; set new STKEND.

        CALL    L2D3B           ; routine INT-TO-FP will read digits till
                                ; a non-digit found.
        CALL    L2DA2           ; routine FP-TO-BC will retrieve number
                                ; from stack at membot.
        JR      C,L1A15         ; forward to E-L-1 if overflow i.e. > 65535.
                                ; 'Nonsense in BASIC'

        LD      HL,$D8F0        ; load HL with value -9999
        ADD     HL,BC           ; add to line number in BC

;; E-L-1
L1A15:  JP      C,L1C8A         ; to REPORT-C 'Nonsense in BASIC' if over.
                                ; Note. As ERR_SP points to ED_ERROR
                                ; the report is never produced although
                                ; the RST $08 will update X_PTR leading to
                                ; the error marker being displayed when
                                ; the ED_LOOP is reiterated.
                                ; in fact, since it is immediately
                                ; cancelled, any report will do.

; a line in the range 0 - 9999 has been entered.

        JP      L16C5           ; jump back to SET-STK to set the calculator 
                                ; stack back to its normal place and exit 
                                ; from there.

; ---------------------------------
; Report and line number outputting
; ---------------------------------
; Entry point OUT-NUM-1 is used by the Error Reporting code to print
; the line number and later the statement number held in BC.
; If the statement was part of a direct command then -2 is used as a
; dummy line number so that zero will be printed in the report.
; This routine is also used to print the exponent of E-format numbers.
;
; Entry point OUT-NUM-2 is used from OUT-LINE to output the line number
; addressed by HL with leading spaces if necessary.

;; OUT-NUM-1
L1A1B:  PUSH    DE              ; save the
        PUSH    HL              ; registers.
        XOR     A               ; set A to zero.
        BIT     7,B             ; is the line number minus two ?
        JR      NZ,L1A42        ; forward to OUT-NUM-4 if so to print zero