Amiga DMA slot timing

Four memory refresh slots ($E2, $01, $03, $05), three disk ($07–$0B), four audio ($0D–$13, one per channel) and sixteen sprite slots ($15–$33, two per sprite: its data words, or POS and CTL at the start) — all odd clocks.

Bitplanes are fetched in units of 8 clocks starting at DDFSTRT while the unit start is at or before DDFSTOP. In lores a unit fetches one word per plane in the order – 4 6 2 – 3 5 1; in hires two words per plane, 4 2 3 1 4 2 3 1. A 320-pixel lores screen ($38–$D0) is 20 units, 160 clocks; 640 hires ($3C–$D4) is 40 words per plane. Fetching can’t start before $18 or continue past $D8.

The 68000 needs a bus cycle of 4 clocks, so it only ever uses even clocks. Up to four lores planes fetch only on odd clocks and cost the CPU nothing; planes 5 and 6 take even ones, so six lores planes steal half of the CPU’s slots in the display (HRM: "160 cycles per line"). Four hires planes take all 160 slots of the window: the 68000 can’t reach chip RAM until the beam reaches the border.

A display that starts fetching earlier than $38 (overscan, horizontal scrolling) takes sprite slots: $30 loses sprite 7, and every 8 clocks earlier loses two more.

The copper uses even clocks only; a MOVE takes 4 clocks (two fetches), a WAIT 6.

The blitter takes any free slot; it has priority over the 68000 but, unless BLTPRI is set, gives it one cycle after three refusals.

AGA (Alice) adds planes 7 and 8 and FMODE: with 32- or 64-bit fetches a fetch unit covers 2× or 4× as many pixels, so the same screen needs a half or a quarter of the slots.

PAL lines are 227 clocks and NTSC lines alternate 228 and 227. At 3.546895 MHz (PAL) and 3.579545 MHz (NTSC) colour clock that is 50.08 and 59.94–60.05 frames per second.

It is a slot-ownership model, not a cycle-exact emulator: the copper’s WAIT wake-up, the blitter’s own idle cycles (which depend on the channels in use and on fill/line mode) and the 68000’s instruction timing are not modelled — the blitter here wants every free slot and the CPU wants every even one.

Sprite loss is shown when a sprite’s slots fall inside the fetch window.

With AGA wide fetches the counts are exact but the position of each fetch inside its unit is drawn approximately (the fetches sit at the end of the unit); AGA sprite and copper timing changes are not modelled.

ECS super hires uses 2-clock units (2 1); productivity and scan-doubled modes are not covered.