LC II vs. 4MB SIMM Problem

Genjoke · Aug 2, 2025

Hi folks.

My first color mac was a 2nd hand LCII I got back in 1995.

A couple years ago I got a new LC II. I got it all the way from France to Brazil, hence I cal him Frenchy (my language has gramatical gender and computers are masculine lol).

I am little by little upgrading it to the dream machine from my youth. It has now:

FPU - I soldered the socket and added a 16MHz 68882
512k VRAM
Blue SCSI
SC-88 MIDI
Assanté SCSI-Ethernet networking (the real deal, not emulated)

I had some 4MB SIMMS but I decided to put them on My Classic II, leaving Frenchy with only 2MB (2x1MM) of extra memory.
Recently (In fact it got here today) I got an Apple Ethernet LC Card and an extra pair of 30 pin 4MB SIMMs.

Here comes the problem: When I install the RAM and start with Extensions Off, I get the 10MB of ram (4 soldered + 8 installed - 2 not addressed). However, if I turn on extensions there's no way it will successfuly boot! I get either:

Freeze during startup before any extension icon is displayed in the bottom of the screen
Illigal instruction
Sad Mac (0000000F / 00007FFF)

I didn't try to install the Ethernet LC Card yet.

I tried even removing the FPU, to no vail. I also tried to boot from 3 different disk images, using system 7.1.2 or 7.5.5. Both started up ok with Extensions Off but could not boot properly when extensions were enabled.

Another symptom: when booted with extensions off and left running with 10MB, I noticed some video artifacts. Tiny pink dots in rows across the screen, very few, but they are there.

Do I have wrong SIMMs? In fact I bought them from a fellow collector who removed them from an IBM 386. They are 9 chips SIMMs.

VRAM Issue (due to the artifacts)?

Do I have any other issue?

Any suggestions?

As always thanks for any help.

jdmcs · Friday at 6:54 PM

The LC II is compatible with standard 30-pin SIMMs that are rated 100ns or faster (-10 or smaller for the timing). The LC II doesn’t do parity, but the extra chip shouldn’t be an issue.

I always suspect memory first: Try running Snooper 2 (not the Emergency Disk version) after booting with extensions disabled. Just select the memory test and let that run. I’m wondering if booting with extensions disabled lets the System load under the bad memory.

If you have a preferred memory test program besides Snooper 2, then by all means use it instead.

Also, a side note that I learned from the technical specifications: “When all SIMM slots are filled with 4MB SIMMs, the lower 2MB of RAM on the logic board cannot be addressed.”

I mention this because I lways thought it was 2MB of SIMM memory that became unaddressable…

Genjoke · 2025-08-16T18:25:05+0100

Think you very much! I’ll try the memory check. I had no idea either that the motherboard memory was the one to be disabled.

Btw, searching the internet trying to sort out the problem I read about the possibility of accessing the 12MB of RAM on an LC by means of an extension… Is that possible? Has anyone ever tried this?

jdmcs · 2025-08-16T20:30:30+0100

Genjoke said:
I read about the possibility of accessing the 12MB of RAM on an LC by means of an extension… Is that possible? Has anyone ever tried this?

I’ve been told that the 10MB limit is in the ASIC itself, which would mean that it would not be possible to overcome the limit with an extension.

There was a PDS accelerator card that allowed more than 10MB of RAM, but that extra RAM was on the PDS card, allowing it to bypass the ASIC on the motherboard.

Search

LC II vs. 4MB SIMM Problem

Genjoke

Tinkerer

jdmcs

TinkerDifferent Board DoP&G

Genjoke

Tinkerer

jdmcs

TinkerDifferent Board DoP&G