z80_unpacker: initial working version with screen-slideshow example

z80_unpacker/example/example.asm

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+;; Example using upkr depacker for screens slideshow
+    OPT --syntax=abf
+    DEVICE ZXSPECTRUM48,$8FFF
+
+    ORG     $9000
+compressed_scr_files:       ; border color byte + upkr-packed .scr file
+    DB      1
+    INCBIN  "screens/Grongy - ZX Spectrum (2022).scr.upk"
+    DB      7
+    INCBIN  "screens/Schafft - Poison (2017).scr.upk"
+    DB      0
+    INCBIN  "screens/diver - Mercenary 4. The Heaven's Devil (2014) (Forever 2014 Olympic Edition, 1).scr.upk"
+    DB      6
+    INCBIN  "screens/diver - Back to Bjork (2015).scr.upk"
+.e:
+
+start:
+    di
+;     OPT --zxnext
+;     nextreg 7,3                 ; ZX Next: switch to 28Mhz
+    ld      ix,compressed_scr_files
+.slideshow_loop
+  ; set BORDER for next image
+    ldi     a,(ix)              ; fake: ld a,(ix) : inc ix
+    out     (254),a
+  ; call unpack of next image directly into VRAM
+    ld      hl,$4000            ; target VRAM
+    exx
+  ; IX = packed data, HL' = destination ($4000)
+  ; returned IX will point right after the packed data
+    call    upkr.unpack
+  ; do some busy loop with CPU to delay between images
+    ld      bc,$AA00
+.delay:
+    .8 ex      (sp),ix
+    dec     c
+    jr      nz,.delay
+    djnz    .delay
+  ; check if all images were displayed, loop around from first one then
+    ld      a,ixl
+    cp      low compressed_scr_files.e
+    jr      z,start
+    jr      .slideshow_loop
+
+  ; include the depacker library, optionally putting probs array buffer near end of RAM
+    DEFINE  UPKR_PROBS_ORIGIN $FA00   ; if not defined, array will be put after unpack code
+    INCLUDE "../unpack.asm"
+
+    SAVESNA "example.sna",start