First public commit.

lenser/coco/viewer.py

@@ -0,0 +1,71 @@
+"""Generates the CoCo cartridge viewer (Motorola 6809 machine code).
+
+Runs from the Program Pak ROM at $C000 (the CoCo autostarts it).  Sets the VDG
+graphics mode via PIA1 $FF22 and the SAM video registers (PMODE 3 and 4 are both
+6144-byte SAM mode 6; only the $FF22 byte differs), copies the 6144-byte image
+from cart ROM down to the video page at $0E00, then holds it.
+
+display: forever (hold), key (poll the keyboard then reset), seconds (count 60 Hz
+field-syncs then reset).  A Program Pak can't cleanly return to BASIC, so key and
+seconds reset the machine (which re-displays the picture).
+"""
+
+from __future__ import annotations
+
+from .mc6809 import Asm
+
+CART_BASE = 0xC000
+VIDEO = 0x0E00
+SCREEN_BYTES = 6144
+RATE = 60                        # NTSC field-syncs per second
+
+
+def build(data_src: int, vdg: int = 0xF8, display: str = "forever",
+          seconds: int = 0) -> bytes:
+    a = Asm(CART_BASE)
+    a.orcc(0x50)                 # mask IRQ + FIRQ; we run standalone
+
+    a.lda_imm(vdg)
+    a.sta_ext(0xFF22)            # VDG graphics mode (PMODE 3 = $E0/$E8, PMODE 4 = $F8)
+
+    # SAM video mode 6 (V2=1, V1=1, V0=0). SAM regs toggle by address; data ignored.
+    a.sta_ext(0xFFC5)            # V2 set
+    a.sta_ext(0xFFC3)            # V1 set
+    a.sta_ext(0xFFC0)            # V0 clear
+
+    # SAM video offset = 7 ($0E00): F0,F1,F2 set; F3..F6 clear.
+    a.sta_ext(0xFFC7); a.sta_ext(0xFFC9); a.sta_ext(0xFFCB)
+    a.sta_ext(0xFFCC); a.sta_ext(0xFFCE); a.sta_ext(0xFFD0); a.sta_ext(0xFFD2)
+
+    # copy SCREEN_BYTES from cart ROM (data_src) to the video page
+    a.ldx_imm(data_src)
+    a.ldu_imm(VIDEO)
+    a.label("copy")
+    a.lda_postinc("x")
+    a.sta_postinc("u")
+    a.cmpu_imm(VIDEO + SCREEN_BYTES)
+    a.bne("copy")
+
+    # ---- hold the picture ----
+    if display == "key":
+        a.clra()
+        a.sta_ext(0xFF02)        # drive all keyboard columns low
+        a.label("kwait")
+        a.lda_ext(0xFF00)        # row sense
+        a.ora_imm(0x80)          # ignore the joystick-compare bit 7
+        a.cmpa_imm(0xFF)
+        a.beq("kwait")           # all rows high -> no key, keep waiting
+        a.jmp_ind(0xFFFE)        # reset
+    elif display == "seconds":
+        a.ldd_imm(max(1, min(0xFFFF, int(seconds) * RATE)))
+        a.label("swait")
+        a.lda_ext(0xFF03)        # PIA0 CB1 (field-sync) status
+        a.bpl("swait")           # bit 7 clear -> no new field yet
+        a.lda_ext(0xFF02)        # read PB data to clear the field-sync flag
+        a.subd_imm(1)
+        a.bne("swait")
+        a.jmp_ind(0xFFFE)        # reset
+    else:                        # forever
+        a.label("hang")
+        a.bra("hang")
+    return a.resolve()