System software 6.0.x is a 24-bit operating system. When a Macintosh system is running system software 6.0.x, the Macintosh is in 24-bit mode. In 24-bit mode, the Macintosh II family of computers has the address space starting at $00 0000 through $7F FFFF (8MB) reserved for system RAM. The SIMM sockets use the entire system RAM address range of $00 0000 through $7F FFFF, which equals 8MB of RAM.
As you can see from this, with 24-bit addressing, installing four 4MB SIMMs would give you 8MB of usable RAM. The memory configuration under "About This Macintosh..." (About the Finder with System 6) will report 16MB installed, but 8MB is usable with the remainder being assigned to the system software. The extra memory assigned to the system software is unusable. In the case of the Macintosh LC, 2MB RAM is on the logic board. If you install two 4MB SIMMs, the Finder will report that 10MB of RAM installed. 8MB of RAM is usable.
With System 7, you have the option of turning on 32-bit addressing in 32-bit clean ROM computers, with the exception of the AV and Power Macintosh models which operate in 32-Bit mode at all times. (MODE32 installed on computers running System 7 versions prior to 7.1 or the 32-Bit System Enabler installed on computers running 7.1 will enable 32-bit operation). This allows addressing above 8MB in those computers that physically support more than 8MB of RAM.
A/UX is also a 32-bit operating system that can address above 8 megabytes of RAM. A/UX supports up to 256MB of physical RAM.
In 32-bit mode, the Macintosh II family of computers (which includes the Macintosh SE/30) has the address space starting at $0000 0000 through $3FFF FFFF (1 gigabyte) reserved for system RAM. NuBus RAM cards may use address $0000 0000 through $3FFF FFFF to add system RAM. The SIMM sockets use address $0000 0000 through $07FF FFFF, which equals 128MB of RAM.
Here's a list of the current Macintosh systems and the maximum physical memory that the SIMM sockets support:
Macintosh System Maximum Physical
RAM
Non 32-Bit clean systems
Macintosh Plus 4MB
Macintosh SE 4MB
Macintosh SE/30 128MB + #
Macintosh II 32MB * ##
Macintosh IIx 32MB * #
Macintosh IIcx 128MB + #
Macintosh Portable 9MB #
Macintosh Portable (backlit) 8MB
32-Bit Clean systems
Macintosh Classic 4MB
Macintosh Classic II 10MB
Macintosh Color Classic 10MB
Macintosh LC 10MB
Macintosh LC II 10MB
Macintosh LC III 36MB
Macintosh LC 475 36MB
Macintosh LC 520 12MB
Macintosh LC 550 36MB
Macintosh LC 575 36MB
Macintosh LC 630 36MB
Macintosh IIci 128MB +
Macintosh IIsi 65MB +
Macintosh IIfx 128MB +
Macintosh IIvi 20MB
Macintosh IIvx 68MB
Macintosh Quadra 610 68MB
Macintosh Quadra 630 36MB
Macintosh Quadra 650 132MB
Macintosh Quadra 660AV 68MB
Quadra 700 68MB +
Quadra 900 256MB +
Quadra 800 136MB
Quadra 840AV 128MB
Quadra 950 256MB +
Power Macintosh 6100/60 72MB
Power Macintosh 7100/66 136MB
Power Macintosh 8100/80 264MB
Macintosh Performa 200 10MB
Macintosh Performa 400 10MB
Macintosh Performa 405 10MB
Macintosh Performa 410 10MB
Macintosh Performa 430 10MB
Macintosh Performa 450 36MB
Macintosh Performa 460 36MB
Macintosh Performa 466/467 36MB
Macintosh Performa 475 36MB
Macintosh Performa 476 36MB
Macintosh Performa 550 36MB
Macintosh Performa 600 68MB
Macintosh Performa 63X 36MB
PowerBook 100 8MB
PowerBook 140 8MB
PowerBook 145 8MB
PowerBook 160 14MB
PowerBook 165 14MB
PowerBook 165c 14MB
PowerBook 170 8MB
PowerBook 180 14MB
PowerBook 180c 14MB
PowerBook Duo 210 24MB
PowerBook Duo 230 24MB
PowerBook Duo 250 24MB
PowerBook Duo 270c 32MB
PowerBook Duo 280c 40MB
# Requires the 32-Bit System Enabler with System 7.1 or MODE32 with versions of System 7 earlier than 7.1.
## Requires the 32-Bit System Enabler with System 7.1 or MODE32 with versions of System 7 earlier than 7.1, as well as the FDHD upgrade kit and PMMU (The FDHD upgrade has been discontinued).
+ These models are designed to handle 16 megabyte SIMMs but only 4 megabyte SIMMs are officially supported by Apple at this time.
* There are two different reasons that the Macintosh II and the Macintosh IIx can't support the higher density SIMMs. First, the Macintosh II ROM startup code doesn't know about 4MB SIMMs and won't start up.
Second, the Macintosh IIx ROM does know about 4MB SIMMs, but standard 4MB SIMMs won't work on the Macintosh IIx. This is because JEDEC, the committee overseeing the standardization of new solid-state devices, added an additional built-in test mode to high-density DRAMs. The test mode is invoked by a sequence of electrical signals which was ignored by earlier-generation DRAM. This specification for 4MB SIMMs was changed after the Macintosh IIx was developed. At the time the Macintosh IIx was developed, in theory, 4MB SIMMs should have worked. It wasn't known at the time that the specification would be changed. The result is that the current standard 4MB SIMMs don't work on the Macintosh IIx.
There are special 4 Megabyte SIMMs available that have a PAL chip which will allow their use in the Macintosh II and IIx. You'll need to contact the SIMM vendor or manufacturer to see if they have these special SIMMs available.
This subject is described in detail in Tech Note #176.
The following Tech Info Library article can help you find the Tech Note mentioned here:
Article 24493: "
Apple Tech Notes: What They Are, Where To Find Them"