Ideas on non-working Mega 4?

[troed]

Acquired a (known) non-working Mega 4 today. Won't have time to open it up and look at for two weeks, but I thought I should post what it looks like when booting for some ideas on the most likely cause It's somewhat interesting.

non_working_mega4.JPG (180.9 KiB) Viewed 4886 times

So, GLUE produces a stable image and the Shifter at least believes it's doing something of value. Not really sure what data it's getting fed with though (but I have a nagging feeling I've seen exactly this issue in a post somewhere, at some time).

The same pattern is repeated, each line, four times. The pattern is 40 bytes in size.

edit: Oh, it's not just a visual problem - it hangs like this. However, it does so at 50Hz so TOS has gotten a few things done first.

[PhilC]

One of mine did something like this with a bad blitter.

[frank.lukas]

test cartridge + null modem cable (9600Baud8N1)

[exxos]

Isn't that the pattern used by TOS when trying to size RAM ?

[troed]

Isn't that the pattern used by TOS when trying to size RAM ?

I was actually reading through the TOS code before to see if it was The below code could possibly do it, although it's not an exact 40 byte pattern. However, at this point screen memory is at $10000 and there's no way I can see for this code to overwrite that address.

Code: Select all

/*
 * We now need to size the actual memory present.  We test memory,
 * starting at 128K and increasing by 128K each time.  For each area of
 * memory, we start at the current address, and work downwards, storing
 * a series of values; we then verify that the values have been stored
 * correctly.  An invalid value (or a bus error when we try to access a
 * memory location) causes us to stop and size the memory accordingly.
 */

        lea.l     $00008000,a7
        movea.l   (_buserror).w,a4
        lea.l     st_ramsize_done(pc),a0 /* bus error means end of ram */
        move.l    a0,(_buserror).w
        move.w    #$FB55,d3           /* d3.w = difference between adjacent stored values */
        move.l    #$00020000,d7       /* d7 = address of first memory test zone & gap between memory test zones */

        movea.l   d7,a0               /* a0 = address of current test zone */
next_zone:
        movea.l   a0,a1               /* a1 -> memory being tested */
        move.w    d0,d2               /* d2.w = test value */
        moveq.l   #42,d1              /* d1.w = loop count-1 */
store_mem:
        move.w    d2,-(a1)            /* store a value */
        add.w     d3,d2               /* update the value for next time */
        dbf       d1,store_mem        /* rinse & repeat */
/* now check the values we stored */
        movea.l   a0,a1               /* a1 -> test area */
        moveq.l   #42,d1              /* loop count */
check_mem:
        cmp.w     -(a1),d0
        bne.s     st_ramsize_done   /* miscompare, ram invalid, exit */
        clr.w     (a1)                /* else zero it out */
        add.w     d3,d0               /* update for next compare */
        dbf       d1,check_mem        /* until we're done */
        adda.l    d7,a0               /* point to next test area */
        bra.s     next_zone
st_ramsize_done:                    /* bus error or bad ram  */
        suba.l    d7,a0               /* (a0 -> first zone that failed) */
        move.l    a0,d5               /* d5 = st ram size */
        move.l    a4,(_buserror).w

test cartridge + null modem cable (9600Baud8N1)

Yeah I must confess I don't think I have a serial cable ... :/

[troed]

test cartridge

Since TOS does manage to set 50Hz I quickly checked whether there was any difference booting a test cart instead, but no. This ... confuses me a bit.

[exxos]

This is where diagrom comes in useful, as it can test out RAM without TOS needed...

[troed]

Solved: Needs a new GLUE.

[troed]

trivia: Hadn't seen this hack before.

megast_psu.JPG (403.36 KiB) Viewed 4722 times

[PhilC]

Bridge rectifier the old fashioned way