exxos's Atari, Amiga & retro forum

SteveBagley wrote: 25 Sep 2025 00:28 More interestingly, is this comment:

Code: Select all

; 12-Feb-1992   ml 5.0c Modified _do_rw() to use the 14-bit counter on the
;                       ACSI DMA chip of the Sparrow.

which more or less confirms there's an 8MB DMA limit. Interestingly, the ST is limited to 128KB transfers according to the same source. If more than 8MB (128KB on the ST) is requested for transfer then the transfer is split into multiple DMA requests.

dml wrote: 25 Sep 2025 07:59 Aside from AHDI, is it only HDDriver & EmuTOS affected, or not known yet?

dml wrote: 25 Sep 2025 07:59 Aside from AHDI, is it only HDDriver & EmuTOS affected, or not known yet?

exxos wrote: 25 Sep 2025 10:42 Jookies test program doesn't use any driver.

dml wrote: 25 Sep 2025 10:46 Does the program perform read & write or just reads? Is there any way for the program to know the DMA address counter is still aligned with the end of the transfer? It may be this problem is not easy to test for until the exact mis-behaviour is understood.

dml wrote: 25 Sep 2025 10:46Does the program perform read & write or just reads? Is there any way for the program to know the DMA address counter is still aligned with the end of the transfer?

#define mfpGpip			((volatile uint8_t *) 0xFFFA01)
#define IO_DINT     0x20        // DMA interrupt (FDC or HDC)

uint8_t wait_dma_cmpl(uint32_t t_ticks)
{
	uint32_t now, until;
	uint8_t gpip;

	now = *HZ_200;
	until = t_ticks + now;              // calc value timer must get to

	while(1) {
		gpip = *mfpGpip;

		if ((gpip & IO_DINT) == 0) {    // Poll DMA IRQ interrupt
			return OK;                 	// got interrupt, then OK
		}

		now = *HZ_200;

		if(now >= until) {
			break;
		}
	}

	return 0xff;                   // no interrupt, and timer expired,
}

stephen_usher wrote: 25 Sep 2025 12:52 I know that this is not relevant to Jookie’s test but it should be made clear that:

Size of file != Size of transfer.

I’m very surprised that Atari drivers are trying to send such large chunks all in the one transfer. It’s very unusual.

regarding your SCSI issue, while preparing and testing the next HDDRIVER
release I played a bit with HDDRUTIL's "Read Test" functionality. This test
reads quite large (in Atari terms) amounts of data with a single SCSI command.
Usually (but this may depend on the HDDRUTIL release used) the test tries
to transfer up to 5 MiB of data in one go. This works fine with my Falcon.
But 10 MiB in one go did *not* work.

With my Ataris I use SCSI2Pi (https://www.scsi2pi.net/), which is perfect
for conveniently debugging scenarios like this on the Pi command line. Using
a nested intervals approach I found that with my Falcon the maximum amount
of bytes that are correctly transferred are 8388096. For a drive with 512
physical bytes per sector this is equivalent to 16383 sectors.

This is an excerpt from an SCSI2Pi log on debug level with 16384 ($4000)
sectors:

[s2p] (ID 3) [debug] Controller is executing READ(10), CDB 28:00:00:00:00:00:00:40:00:00
[s2p] (ID:LUN 3:0) [debug] Device is executing READ(10) ($28)
[s2p] (ID 3) [warning] Sent 33 byte(s), 512 required
[s2p] (ID 3) [warning] Received RESET signal

Something is obviously wrong here, HDDRIVER had to reset the SCSI bus.

With 16383 ($3fff) sectors there are no issues:

[s2p] (ID 3) [debug] Controller is executing READ(10), CDB 28:00:00:00:00:00:00:3f:ff:00
[s2p] (ID:LUN 3:0) [debug] Device is executing READ(10) ($28)
[s2p] (ID 3) [debug] Controller is executing READ(10), CDB 28:00:00:00:3f:ff:00:20:01:00
[s2p] (ID:LUN 3:0) [debug] Device is executing READ(10) ($28)

The 16384 boundary does not appear to be arbitrary, because it is 2^14. I
am not aware of any official Falcon documentation that says there is such
a limit, though. But a final official Falcon documentation does not even
exist I think. Atari probably had no money left do document things .

16384 is very small compared to what the TT supports, or to transfer sizes
used by other SCSI platforms. The SCSI protocol has been updated several
times to support larger transfer sizes, because they are important for
high throughput. Up to 2^32 sectors in one go are supported even by old
SCSI standards, because for obvious reasons workstations prefer large
transfer sizes.

Anyway, there is kind of a conflict now: Without any official source saying
that there is an 8 MiB limit I am reluctant to change the optimized transfer
mode settings in HDDRIVER 12.7 to 8 MiB. Just the fact that I can clearly
show this limit with my Falcon is not really a proof. On the other hand,
with a Falcon it does not really matter whether the limit is 8 MiB or more.
In particular HDDRIVER users appear not to have been affected by this for
decades, which is not that surprising because. transfers of more than 8 MiB
*in one go* are very unusual with an Atari.