Hardware oddity: Dual-Port Hazard

Wuerfel_21

This is something I noticed a long while ago, but here's a proper demo:
Reading a dual port RAM cell at the same time it is written returns an indeterminate value.
(The stored value is fine, it's just a momentary glitch)

CON
_CLKFREQ = 10_000_000 ' Higher speeds are more suceptible
HAZARD_CELL = $004 ' Which cogRAM cell to test - can't go lower than 3 with this

DAT
              org
              long 0[HAZARD_CELL-3]
.outer
              rep @.inner,testlen
              xor .hazard,#31 ' ---\
              nop             '    | XOR result written on same cycle as BITH opcode fetch
.hazard       bith val,#0  ' <-----/
.inner
              cmp val,expect wz
        if_nz jmp #.cought
              add loopctr,#1
              test loopctr,##1023 wz
        if_nz jmp #.outer
              debug("Nothing yet... ",udec(loopctr))
              jmp #.outer

.cought
              debug("Hazard cought! ",ubin_long(val),udec(loopctr),uhex_long(.hazard))
              jmp #$


val           long 0
expect        long $80000001
loopctr       long 0
testlen       long 1024

              fit 496

You may need to try different HAZARD_CELL values or increase _CLKFREQ to get a hit.
It should also be possible to reproduce this with LUT (only streamer and pair sharing use the 2nd LUT port, so it's harder to run into this).

Rayman

Guess one need two NOPs with self-modifying code to be safe?

Wuerfel_21

@Rayman said:
Guess one need two NOPs with self-modifying code to be safe?

Yes, but that's documented and (I hope) well known.
(it most usually will give you the old value, so there'd be a bug, anyways)

Rayman

I was looking around for such "documentation" but couldn't find anything on self-modifying code... Have you seen it somewhere?

Wuerfel_21

@Rayman said:
I was looking around for such "documentation" but couldn't find anything on self-modifying code... Have you seen it somewhere?

P2 silicon doc, but I don't think where I originally got it.

Rayman

Ok, thanks. Know what happened... Tried to use the Edge search, but that doesn't work in Google Docs, have to use their search

Still seems like there should be a section titled "Sefl-Modifying Code" with that note in it...

Wuerfel_21

New p2docs page: https://p2docs.github.io/errata.html
Featuring all the favorites!

rogloh

@Wuerfel_21 said:
New p2docs page: https://p2docs.github.io/errata.html
Featuring all the favorites!

Good to have all this in the one place. Nice work.