First public commit.

lenser/bbc/viewer/bbc.s

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+; lenser -- BBC Micro viewer (6502), loaded at PAGE (&1900) and *RUN.
+;
+; Sets the screen MODE, programmes the logical->physical palette, then *LOADs the
+; image file "IMG" straight into screen memory, and holds it per the display
+; option.  Parameters come from a fixed block the packager fills in (so the code
+; is constant); see build_viewer() in assemble.py.
+;
+; #defines from the wrapper -- WAITMODE (0 forever, 1 key, 2 seconds), WAITSECS, RATE.
+;
+; assembled by bbc/viewer/assemble.py via xa
+
+OSWRCH = $FFEE
+OSRDCH = $FFE0
+OSBYTE = $FFF4
+OSCLI  = $FFF7
+
+        * = $1900
+start:
+        ; ---- VDU 22, mode ----
+        lda #22
+        jsr OSWRCH
+        lda p_mode
+        jsr OSWRCH
+
+        ; ---- palette via VDU 19, logical, physical, 0,0,0  for each logical ----
+        ldx #0
+palloop:
+        cpx p_ncol
+        beq paldone
+        lda #19
+        jsr OSWRCH
+        txa
+        jsr OSWRCH                ; logical colour = X
+        lda p_pal,x
+        jsr OSWRCH                ; physical
+        lda #0
+        jsr OSWRCH
+        jsr OSWRCH
+        jsr OSWRCH
+        inx
+        bne palloop
+paldone:
+
+        ; ---- hide the text cursor (VDU 23,1,0;0;0;0;) ----
+        lda #23
+        jsr OSWRCH
+        lda #1
+        jsr OSWRCH
+        ldx #8
+curz:
+        lda #0
+        jsr OSWRCH
+        dex
+        bne curz
+
+        ; ---- *LOAD the image straight into screen memory ----
+        ldx #<cmd
+        ldy #>cmd
+        jsr OSCLI
+
+        ; ---- hold the picture ----
+#if WAITMODE == 0
+hang:
+        jmp hang
+#endif
+#if WAITMODE == 1
+        jsr OSRDCH                ; block until a key
+        rts                       ; back to BASIC
+#endif
+#if WAITMODE == 2
+        lda #<(WAITSECS*RATE)
+        sta cnt
+        lda #>(WAITSECS*RATE)
+        sta cnt+1
+swait:
+        lda #19
+        jsr OSBYTE                ; OSBYTE 19 = wait for vertical sync
+        lda cnt
+        bne sdec
+        dec cnt+1
+sdec:
+        dec cnt
+        lda cnt
+        ora cnt+1
+        bne swait
+        rts                       ; back to BASIC
+#endif
+
+cnt:    .byte 0,0
+
+; OSCLI string -- the packager patches in the right screen-base hex
+cmd:    .byte "LOAD IMG ", "0000", 13
+
+; parameter block -- the packager fills these in
+p_mode: .byte 0
+p_ncol: .byte 0
+p_pal:  .byte 0,0,0,0,0,0,0,0