System 6.0.x, 7.x: How They Handle 4MB SIMMs

If a SIMM bank is filled with 4MB modules (giving 16MB), will 8MB be used by System Software 6.0.x, or will the system not know what to do with these SIMMs? We are concerned that these SIMMs will bomb the system.
This article has been archived and is no longer updated by Apple.
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

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"
Published Date: Feb 18, 2012