Blitter & Minterms

Blitter & Minterms

The Amiga’s blitter combines up to three source channels (A, B, C) into a destination (D) using a minterm — an 8-bit boolean function. Black Crypt uses a small set of them, and knowing them tells you how the data is laid out.

The minterms

Minterm	Meaning	Used for
$0FCA	D = (A AND B) OR (NOT A AND C)	mask + colour sprite blit
$09F0	D = C	straight screen-to-screen copy
$03CA	D = B	opaque source-to-screen copy (no mask)
$00F0	D = C	full word fill/copy

The mask + colour blit ($0FCA)

This is the workhorse. It draws a masked sprite onto the screen:

• Channel A = the transparency mask (1 = pixel, 0 = transparent), fixed per plane loop.
• Channel B = the colour data, advancing by the plane stride each plane.
• Channels C/D = the screen (read/write), same pointer.

The result: where the mask is 1, the colour plane is written; where the mask is 0, the screen is left untouched. This is why the 7-plane format (mask + 6bpp colour) is so common — the blitter consumes it directly.

The main sprite blitter

The game’s central blitter routine (LAB_011E) does exactly this:

• Minterm $0FCA: A = mask (fixed), B = colour (stride), C/D = screen.
• 6 iterations (one per colour bitplane).
• Two paths: a clipped path (flag bit 0) for sprites near the screen edge, and a fast path otherwise.

BLTSIZE encoding

The blitter’s size register packs height and width:

BLTSIZE = (height << 6) | width_in_words

So $0603 = height 24, width 3 words (24 pixels). $0211 = height 8, width 17 words (272 pixels).

Register & LVO conventions

When reading the disassembly:

• A6 = library base (dos, exec, graphics), A5 = local data frame, A4 = overlay data.
• DOS LVO offsets: Open=-30, Close=-36, Read=-42, Write=-48, Lock=-84.
• 12-bit Amiga RGB → 24-bit: multiply each nibble by 17 (0xC86 → 204,136,102).
• EHB half-bright (32–63) = (r>>1, g>>1, b>>1).