Instruction Hazards
Interlocked delay stalls the pipeline when a result is required too early. Non-interlocked hazard requires instruction scheduling in microcode. Special registers are never forwarded.
| Producer → consumer | Required interval / effect | Source |
|---|---|---|
| Memory load → dependent instruction | Interlocked delay 1 | p.9 |
| Scalar multiply → dependent instruction | Delay 1 | p.8 |
| Shift result → branch decision | Interlocked delay 1 | p.8 |
Write Flag, Timer or Seg → read/use | Hazard 3 | pp.6–7, 25–26 |
clip / clipv → Clip read | Hazard 1 | p.7 |
sort → Seg / Clip read | Seg hazard 3; Clip hazard 2 | p.9 |
| Seed operation → Scale / seed index / Seed | Scale and index hazard 1; Seed hazard 2 | p.7 |
Write Triangle Engine shadow destination → spr / spri / sprii | Hazard 2 | p.9 |
step Cnt / stepv Cnt → draw | Hazard 1 | p.12 |
| Set XShadow → parallel draw | Hazard 1; no such hazard for parameterized draw | pp.13–14 |
| Write DrawCtl → compatibility draw | Hazard 3 | pp.13–14 |
| Enable V2000 mode → set shadow registers | No hazard specified | p.16 |
tri → Fill Engine instruction | Hazard 3 | p.16 |
| Fill instruction → seed-generating instruction | Three-instruction hazard; seed descriptions say to issue more than three instructions later | pp.8, 12, 22 |
Fill instruction → draw / rdraw | At least one separating instruction | p.22 |
Fill parameter writes have their own hazards: format and offset 3, write mask 1, color/key 2, brush or source data 3. See Fill parameter registers.
The PDF sometimes describes intervals as cycles and sometimes as instructions. The table preserves the supplied values; instruction launch time and engine completion time are listed separately in opcode details.
Engine stalls
Writing DrawCtl, a0..a31 or q0..q15 while the Triangle Engine is busy stalls the RISC until it becomes idle. A triangle or parameterized draw reached while Fill is busy also waits. Fill waits for Pixel and Triangle idle before launch.
During FIFO-sourced fills, InputFIFO reads and shifter instructions stall until the fill completes. mbltfo also stalls writes to a8. Most fill parameters can otherwise be overwritten after launch because they are shadowed or consumed at startup.
Sources: Technical Spec, p.6, Technical Spec, p.7, Technical Spec, p.8, Technical Spec, p.9, Technical Spec, p.12, Technical Spec, p.13, Technical Spec, p.14, Technical Spec, p.16, Technical Spec, p.22, Technical Spec, p.24