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---
title: DMA List Loops
type: test
summary: Times CPU work while DMA channel 2 walks a finished list, a list that points at itself and two entries that point at each other.
category: Hardware tests
info: Machine = SCPH-5501
info: Program = dma/chain-looping (ps1-tests)
---

## Overview

The work is a loop of 1000 additions to a variable in RAM. Root counter 2 times it in units of 8 system clock cycles, and the program prints the time in cycles. The program times it once with no DMA running. It then starts [DMA](/hardware/dma/) channel 2 in list mode (sync mode 2) on three lists: a list with one entry that ends at once, a list whose only entry points to itself, and two entries that point to each other.[^src] After the work the program reads whether CHCR bit 24 has cleared (finished) and whether the channel's DICR flag is set (irq), then stops the channel.[^src]

## Results

The work took 17,104 to 17,120 cycles with no DMA running and the same with the finished list. That list finished and raised no interrupt flag.[^run]

With the self-referencing list and with the pair of entries the work took 25,648 to 25,672 cycles. That is about 50% longer than the work alone. Neither list finished, and neither raised an interrupt flag.[^run]

Table: Work time in system clock cycles over six runs

| List | Work | Finished | IRQ |
| --- | --- | --- | --- |
| No DMA | 17,104 to 17,120 | | |
| One entry that ends | 17,104 to 17,120 | yes | no |
| One entry pointing to itself | 25,648 to 25,664 | no | no |
| Two entries pointing to each other | 25,648 to 25,672 | no | no |

## Runs

### 25 September 2026

[12CHN.txt](/tests/raw/20260925T0000Z-5501a-card-main/main/12CHN.txt)

```
===== 12 CHN DMA CHAIN LOOP | BIOS 19951204 System ROM Version 3.0 11/18/96 A

dma/chain-looping
Tests CPU behavior with various invalid GPU linked list chains.

Testing with no DMA active...
  Work took 17120 ticks
Testing with empty but complete chain...
  Work took 17112 ticks, finished = true, irq = false
Testing single self-referencing chain...
  Work took 25648 ticks, finished = false, irq = false
Testing double self-referencing chain...
  Work took 25656 ticks, finished = false, irq = false
```

### 27 September 2026

[12CHN.txt](/tests/raw/20260927T0000Z-5501a-card-2/main/s024/12CHN.txt)

```
===== 12 CHN DMA CHAIN LOOP | BIOS 19951204 System ROM Version 3.0 11/18/96 A

dma/chain-looping
Tests CPU behavior with various invalid GPU linked list chains.

Testing with no DMA active...
  Work took 17104 ticks
Testing with empty but complete chain...
  Work took 17104 ticks, finished = true, irq = false
Testing single self-referencing chain...
  Work took 25664 ticks, finished = false, irq = false
Testing double self-referencing chain...
  Work took 25664 ticks, finished = false, irq = false
```

### 27 September 2026 run 2

[12CHN.txt](/tests/raw/20260927T0000Z-5501a-card-b/main/s034/12CHN.txt)

```
===== 12 CHN DMA CHAIN LOOP | BIOS 19951204 System ROM Version 3.0 11/18/96 A

dma/chain-looping
Tests CPU behavior with various invalid GPU linked list chains.

Testing with no DMA active...
  Work took 17112 ticks
Testing with empty but complete chain...
  Work took 17120 ticks, finished = true, irq = false
Testing single self-referencing chain...
  Work took 25664 ticks, finished = false, irq = false
Testing double self-referencing chain...
  Work took 25672 ticks, finished = false, irq = false
```

### 27 September 2026 run 3

[12CHN.txt](/tests/raw/20260927T0000Z-5501a-card-g/main/12CHN.txt)

```
===== 12 CHN DMA CHAIN LOOP | BIOS 19951204 System ROM Version 3.0 11/18/96 A

dma/chain-looping
Tests CPU behavior with various invalid GPU linked list chains.

Testing with no DMA active...
  Work took 17104 ticks
Testing with empty but complete chain...
  Work took 17104 ticks, finished = true, irq = false
Testing single self-referencing chain...
  Work took 25664 ticks, finished = false, irq = false
Testing double self-referencing chain...
  Work took 25664 ticks, finished = false, irq = false
```

### 27 September 2026 run 4

[12CHN.txt](/tests/raw/20260927T0000Z-5501a-card-g3/main/12CHN.txt)

```
===== 12 CHN DMA CHAIN LOOP | BIOS 19951204 System ROM Version 3.0 11/18/96 A

dma/chain-looping
Tests CPU behavior with various invalid GPU linked list chains.

Testing with no DMA active...
  Work took 17104 ticks
Testing with empty but complete chain...
  Work took 17112 ticks, finished = true, irq = false
Testing single self-referencing chain...
  Work took 25648 ticks, finished = false, irq = false
Testing double self-referencing chain...
  Work took 25656 ticks, finished = false, irq = false
```

### 28 September 2026

[12CHN.txt](/tests/raw/20260928T0000Z-5501a-card-h/main/12CHN.txt)

```
===== 12 CHN DMA CHAIN LOOP | BIOS 19951204 System ROM Version 3.0 11/18/96 A

dma/chain-looping
Tests CPU behavior with various invalid GPU linked list chains.

Testing with no DMA active...
  Work took 17104 ticks
Testing with empty but complete chain...
  Work took 17104 ticks, finished = true, irq = false
Testing single self-referencing chain...
  Work took 25648 ticks, finished = false, irq = false
Testing double self-referencing chain...
  Work took 25648 ticks, finished = false, irq = false
```

## References

[^src]: ps1-tests, dma/chain-looping. https://github.com/JaCzekanski/ps1-tests.
[^run]: SCPH-5501 output: [main/12CHN.txt](/tests/raw/20260925T0000Z-5501a-card-main/main/12CHN.txt) and the files beside it, with the other runs listed under Runs.