DMA Start Stall and Delay Slot Fetch

Measures when the CPU waits for an OTC transfer started by a CHCR store, and the time of a call whose delay slot begins a new cache line.

Overview

The program runs two groups.¹ DSK stores to the CHCR of DMA6 to start an OTC transfer of 1, 4 or 16 words and reads timer 2 k NOPs later. Each entry gives the cycles from the store to the read with the channel enabled in DPCR and then with it disabled, and each row runs three times.² FSL zeroes timer 2 in KSEG1 code, makes a call by jalr and reads the timer after the return. The jalr sits at word 3 of a line, so that its delay slot begins the next line, or at word 0, and the target is cached or in KSEG1.³

See also:

• DMA Priority Against the CPU and Between Channels

Results

OTC Start Stall

With the channel disabled the read came k + 1 cycles after the store from k 1 on. With the channel enabled and 0 to 3 NOPs, the read came at the same time as with it disabled. With 4 or more NOPs the read waited for the transfer. The wait ended at 11 cycles for 1 word and at 14 cycles for 4 words. For 16 words it ended at 26 cycles. Once the NOPs took longer than that, the enabled and the disabled times were the same again.²

The first run of the 1-word transfer took longer with the channel enabled: 7 cycles at k 0 and 1, 22 at k 3 to 8, 44 at k 16 and 84 at k 32. The 16-word transfer took 29 and 36 cycles in one run each.²

Delay Slot at a Line Start

With a cached target, the call took 25 cycles cold when the jalr was at word 3 and 26 cycles when it was at word 0, and 11 cycles warm in both places. With a target in KSEG1 it took 30 and 31 cycles cold and 16 cycles warm.³

Runs

8 October 2026

followup35/00RUN.txt

===== 00 RUN | BIOS 19951204 System ROM Version 3.0 11/18/96 A
2 tests
Saved on the card in slot 2, from image 1
COP0 at start: BPC 00000000 BDA 00000000 TAR BFC02B68 DCIC 00000000 BadVaddr 00000480 BDAM 00000000
BPCM 00000000 SR 00000000 Cause C0000024 EPC BFC02B78 PRId 00000002

followup35/01DSK.txt

===== 01 DSK F35 DMA STOP | BIOS 19951204 System ROM Version 3.0 11/18/96 A
BadVaddr as the group starts: 00000480
Clocks from a CHCR store to a timer read k NOPs on, OTC, the channel enabled / disabled
in DPCR, three times each (row 240):
OTC  1 words, k  0: 7/2 2/2 2/2
OTC  1 words, k  1: 7/2 2/2 2/2
OTC  1 words, k  2: 7/3 3/3 3/3
OTC  1 words, k  3: 22/4 4/4 4/4
OTC  1 words, k  4: 22/5 11/5 11/5
OTC  1 words, k  8: 22/9 11/9 11/9
OTC  1 words, k 16: 44/17 17/17 17/17
OTC  1 words, k 32: 84/33 33/33 33/33
OTC  4 words, k  0: 2/2 2/2 2/2
OTC  4 words, k  1: 2/2 2/2 2/2
OTC  4 words, k  2: 3/3 3/3 3/3
OTC  4 words, k  3: 4/4 4/4 4/4
OTC  4 words, k  4: 14/5 14/5 14/5
OTC  4 words, k  8: 14/9 14/9 14/9
OTC  4 words, k 16: 17/17 17/17 17/17
OTC  4 words, k 32: 33/33 33/33 33/33
OTC 16 words, k  0: 2/2 2/2 2/2
OTC 16 words, k  1: 2/2 2/2 2/2
OTC 16 words, k  2: 3/3 3/3 3/3
OTC 16 words, k  3: 4/4 4/4 4/4
OTC 16 words, k  4: 26/5 29/5 26/5
OTC 16 words, k  8: 36/9 26/9 26/9
OTC 16 words, k 16: 26/17 26/17 26/17
OTC 16 words, k 32: 33/33 33/33 33/33
Done.

followup35/02FSL.txt

===== 02 FSL F35 FILL SLOT | BIOS 19951204 System ROM Version 3.0 11/18/96 A
BadVaddr as the group starts: 00000480
Timer 2 from its zeroing in a KSEG1 shell to lhu of it after a call and its return,
cold (FlushCache first) three times, then warm (row 241):
jalr at word 3, slot begins a line, target cached: cold 25 25 25, warm 11
jalr at word 3, slot begins a line, target KSEG1: cold 30 30 30, warm 16
jalr at word 0, slot word 1, target cached: cold 26 26 26, warm 11
jalr at word 0, slot word 1, target KSEG1: cold 31 31 31, warm 16
Done.

References