ST536 update: the warm reset vs fast ROM problem, and what I have done about it
This one has been a bit of a double-edged sword, so here is the whole story in plain terms.
How fast ROM works on the ST536
When TOS boots, it copies the ROM into TT-RAM. Once the copy is done, the PLD sends every ROM access to the TT-RAM copy instead of the real ROM chips, which is a lot faster. The PLD remembers that the copy has been done, and that memory survives a reset.
Problem 1
Because that "copy done" flag survives a reset, the machine keeps running TOS from the TT-RAM copy after you press reset. If that copy ever got corrupted, pressing reset did not help. You got the green line at the top of the screen and nothing else, and only powering off for 10 seconds or more brought it back.
Problem 2
The obvious fix is to clear the flag on reset so TOS runs from the real ROM again. The catch is that the ROM copy relies on the TT-RAM check, which TOS only does on a full cold boot. A warm reset skips all that, so after pressing reset you were left running from the real ROM at stock speed until the next power off.
Problem 3
I tried clearing the flag on every reset anyway, and for reasons I have not got to the bottom of yet, the machine then would not boot at all. So that was no good either.
So the menu was: slow ROM after a reset, a machine that might not come back after a reset, or a machine that did not boot at all. Not a great menu.
Why a reset was always a warm boot
The ST536 keeps the bottom part of RAM, where TOS keeps its system variables, in TT-RAM rather than ST RAM. That includes the magic number TOS checks on every boot to decide between a warm boot and a cold boot. TT-RAM keeps its contents through a reset, so TOS always found the magic number and always did a warm boot.
The fix
It is monumentally easier to just make TOS do a cold boot every time.
- After a reset, the first time TOS reads that magic number, the PLD briefly disconnects the TT-RAM from the CPU for that single read.
- TOS does not see the number it is looking for, decides it is a cold boot, and does everything a cold boot does: memory test, TT-RAM check and the ROM copy.
- Once TOS writes the number back, everything carries on as normal until the next reset.
- The SDRAM controller itself is untouched, as it is very fragile to "innocent" changes.
- No TOS changes were needed, so no new ROMs to burn either.
The downside
This will annoy some people, but there is no such thing as a warm reset on the ST536 any more. Every reset is a cold boot, so anything that normally survives a reset, like RAM disks or reset-resident programs, will not. Reset also takes a little longer because of the memory test. Think of the reset button as the "have you tried turning it off and on again" button, minus the turning it off bit.
Where it stands
First tests on my H4 look good. Reset gives a full cold boot and fast ROM is still there after every reset. Power off boots are fine too. As a bonus, the timings on the latest build actually came out slightly better than before.
One thing which may not be obvious: because the "copy done" flag still survives a reset, the "ROM copy" you see after pressing reset is not reading the real ROM chips at all. TOS is reading the TT-RAM copy and writing it straight back over itself, which is why it is so quick. The real ROM chips are only read again after a power off.
To do
- Make the cold boot after a reset copy from the real 16-bit ROM chips into TT-RAM again, instead of copying the TT-RAM copy back over itself. That way, if there is ever an SDRAM problem, the reset button gets you a fresh copy straight from the ROM chips. If the copy or its check fails, TOS never switches over to TT-RAM and simply keeps running from the 16-bit ROM, so either way you always end up running a good copy of ROM.
- More testing, including on the newer board revision.
- The green line after a very short power off, which is a separate thing still on my list.
