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t/data/zx81.asm  view on Meta::CPAN

; ---

        defb $FF, $FF, $FF      ; unused locations.
                                ; 
; ---------------------------------------
; THE 'FLOATING POINT CALCULATOR' RESTART
; ---------------------------------------
; this restart jumps to the recursive floating-point calculator.
; the ZX81's internal, FORTH-like, stack-based language.
;
; In the five remaining bytes there is, appropriately, enough room for the
; end-calc literal - the instruction which exits the calculator.

;; FP-CALC

FP_CALC:
        jp CALCULATE            ; jump immediately to the CALCULATE routine.
                                ; 

; ---

;; end-calc

end_calc:
        pop af                  ; drop the calculator return address RE-ENTRY
        exx                     ; switch to the other set.
                                ; 
        ex (sp), hl             ; transfer H'L' to machine stack for the
                                ; return address.
                                ; when exiting recursion then the previous
                                ; pointer is transferred to H'L'.
                                ; 
        exx                     ; back to main set.
        ret                     ; return.
                                ; 
                                ; 

; -----------------------------
; THE 'MAKE BC SPACES'  RESTART
; -----------------------------
; This restart is used eight times to create, in workspace, the number of
; spaces passed in the BC register.

;; BC-SPACES

BC_SPACES:
        push bc                 ; push number of spaces on stack.
        ld hl, (E_LINE)         ; fetch edit line location from E_LINE.
        push hl                 ; save this value on stack.
        jp RESERVE              ; jump forward to continue at RESERVE.
                                ; 

; -----------------------
; THE 'INTERRUPT' RESTART
; -----------------------
;   The Mode 1 Interrupt routine is concerned solely with generating the central
;   television picture.
;   On the ZX81 interrupts are enabled only during the interrupt routine,
;   although the interrupt
;   This Interrupt Service Routine automatically disables interrupts at the
;   outset and the last interrupt in a cascade exits before the interrupts are
;   enabled.
;   There is no DI instruction in the ZX81 ROM.
;   An maskable interrupt is triggered when bit 6 of the Z80's Refresh register
;   changes from set to reset.
;   The Z80 will always be executing a HALT (NEWLINE) when the interrupt occurs.
;   A HALT instruction repeatedly executes NOPS but the seven lower bits
;   of the Refresh register are incremented each time as they are when any
;   simple instruction is executed. (The lower 7 bits are incremented twice for
;   a prefixed instruction)
;   This is controlled by the Sinclair Computer Logic Chip - manufactured from
;   a Ferranti Uncommitted Logic Array.
;
;   When a Mode 1 Interrupt occurs the Program Counter, which is the address in
;   the upper echo display following the NEWLINE/HALT instruction, goes on the
;   machine stack.  193 interrupts are required to generate the last part of
;   the 56th border line and then the 192 lines of the central TV picture and,
;   although each interrupt interrupts the previous one, there are no stack
;   problems as the 'return address' is discarded each time.
;
;   The scan line counter in C counts down from 8 to 1 within the generation of
;   each text line. For the first interrupt in a cascade the initial value of
;   C is set to 1 for the last border line.
;   Timing is of the utmost importance as the RH border, horizontal retrace
;   and LH border are mostly generated in the 58 clock cycles this routine
;   takes .

;; INTERRUPT

INTERRUPT:
        dec c                   ; (4)  decrement C - the scan line counter.
        jp nz, SCAN_LINE        ; (10/10) JUMP forward if not zero to SCAN-LINE
                                ; 
        pop hl                  ; (10) point to start of next row in display
                                ; file.
                                ; 
        dec b                   ; (4)  decrement the row counter. (4)
        ret z                   ; (11/5) return when picture complete to L028B
                                ; with interrupts disabled.
                                ; 
        set 3, c                ; (8)  Load the scan line counter with eight.
                                ; Note. LD C,$08 is 7 clock cycles which
                                ; is way too fast.
                                ; 
; ->

;; WAIT-INT

WAIT_INT:
        ld r, a                 ; (9) Load R with initial rising value $DD.
                                ; 
        ei                      ; (4) Enable Interrupts.  [ R is now $DE ].
                                ; 
        jp (hl)                 ; (4) jump to the echo display file in upper
                                ; memory and execute characters $00 - $3F
                                ; as NOP instructions.  The video hardware
                                ; is able to read these characters and,
                                ; with the I register is able to convert
                                ; the character bitmaps in this ROM into a
                                ; line of bytes. Eventually the NEWLINE/HALT
                                ; will be encountered before R reaches $FF.
                                ; It is however the transition from $FF to
                                ; $80 that triggers the next interrupt.
                                ; [ The Refresh register is now $DF ]
                                ; 

; ---

;; SCAN-LINE

SCAN_LINE:
        pop de                  ; (10) discard the address after NEWLINE as the
                                ; same text line has to be done again
                                ; eight times.
                                ; 
        ret z                   ; (5)  Harmless Nonsensical Timing.
                                ; (condition never met)
                                ; 
        jr WAIT_INT             ; (12) back to WAIT-INT
                                ; 

;   Note. that a computer with less than 4K or RAM will have a collapsed