amiga-bootcamp/08_graphics/blitter.md

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Blitter — DMA Engine, Minterms, BltBitMap

Overview

The Blitter is a DMA engine in the custom chips that performs bulk memory operations: block copies, line drawing, area fills, and arbitrary boolean combinations of up to three source bitmaps. It operates independently of the CPU, freeing the 68k for other work.

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Channels

The blitter has four DMA channels:

Channel	Name	Direction	Description
A	Source A	Read	First source bitmap
B	Source B	Read	Second source (often mask/pattern)
C	Source C	Read	Third source (typically destination for read-modify-write)
D	Destination	Write	Output

Each channel has: pointer register, modulo register, shift register (A/B only), and first/last word masks (A only).

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Minterm Logic

The blitter combines A, B, C inputs using an 8-bit minterm value. Each bit selects whether the output is 1 for a specific combination:

Bit	A	B	C	Common Use
7	1	1	1	—
6	1	1	0	—
5	1	0	1	—
4	1	0	0	—
3	0	1	1	—
2	0	1	0	—
1	0	0	1	—
0	0	0	0	—

Common minterm values:

Minterm	Hex	Operation
$F0	A	Copy A to D (straight copy)
$CA	AB + ~AC	Cookie-cut: A=mask, B=source, C=background
$3C	A XOR C	XOR blit (sprite toggle)
$0F	NOT A	Invert source
$00	0	Clear destination
$FF	1	Fill destination with 1s

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Register Map

Address	Name	Description
$DFF040	BLTCON0	Control: use-channels + minterm + shift-A
$DFF042	BLTCON1	Control: direction, fill mode, line mode
$DFF044	BLTAFWM	First word mask for channel A
$DFF046	BLTALWM	Last word mask for channel A
$DFF048	BLTCPT	Channel C pointer (high word)
$DFF04A	BLTCPT	Channel C pointer (low word)
$DFF04C	BLTBPT	Channel B pointer
$DFF050	BLTAPT	Channel A pointer
$DFF054	BLTDPT	Channel D pointer (destination)
$DFF058	BLTSIZE	Size and start: `(height << 6)
$DFF064	BLTCMOD	Channel C modulo
$DFF062	BLTBMOD	Channel B modulo
$DFF060	BLTAMOD	Channel A modulo
$DFF066	BLTDMOD	Channel D modulo

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OS-Level Blitter Functions

/* graphics.library */

/* Copy rectangular region between bitmaps: */
LONG BltBitMap(
    struct BitMap *srcBM, WORD srcX, WORD srcY,
    struct BitMap *dstBM, WORD dstX, WORD dstY,
    WORD sizeX, WORD sizeY,
    UBYTE minterm,             /* usually $C0 = copy */
    UBYTE mask,                /* plane mask */
    APTR tempA                 /* temp buffer or NULL */
);

/* Blit into RastPort (clips to layer): */
void BltBitMapRastPort(struct BitMap *src, WORD srcX, WORD srcY,
                       struct RastPort *rp, WORD dstX, WORD dstY,
                       WORD sizeX, WORD sizeY, UBYTE minterm);

/* Wait for blitter completion: */
void WaitBlit(void);   /* must call before freeing blit buffers */

/* Gain exclusive blitter access: */
void OwnBlitter(void);
void DisownBlitter(void);

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References

• HRM: Amiga Hardware Reference Manual — Blitter chapter
• NDK39: hardware/blit.h, graphics/gfx.h
• ADCD 2.1: BltBitMap, BltBitMapRastPort, OwnBlitter