Editing Macintosh II (section)

=== Memory ===
The standard memory was 1 [[megabyte]], expandable to 8 MB.<ref>{{cite web| url=http://www.krsaborio.net/research/1980s/88/880919_c.htm |title=Apple Announces 68030 Macintosh IIx With High Density Compatible Drive | archive-url=https://archive.today/20120908022711/http://www.krsaborio.net/research/1980s/88/880919_c.htm |archive-date=September 8, 2012 | author1=John Cook |author2=Carol Cochrane |work=Business Wire |date=19 September 1988 |access-date=September 20, 2009}}</ref> The Mac II had eight 30-pin [[SIMM]]s, and memory was installed in groups of four (called "Bank A" and "Bank B").

The Macintosh II does not have a [[Memory management unit|PMMU]] installed by default. Instead, it relies on the [[memory controller]] hardware to map the installed memory into a contiguous [[address space]]. This hardware has the restriction that the address space dedicated to Bank A must be larger than that of Bank B. Though this memory controller was designed to support 16 Megabyte, 30-pin SIMMs in each available slot (for a total of up to 128 MB of RAM), the original Macintosh II ROMs have problems that limit the amount of RAM that can be installed into each slot to just 8 MB SIMMs. Although the later Macintosh IIx ROMs that shipped with the Macintosh II FDHD upgrade fixes this initial problem, these newer ROMs still do not have a 32-bit memory manager and cannot boot into 32-bit address mode, at least, not without software assistance in the form of "MODE32", thus limiting the ''total'' amount of RAM to a mere 8MB.<ref>{{cite web |url=http://db.tidbits.com/series/1193 |title=Series: The 24-bit ROM Blues |author=Adam C. Engst |website=[[Tidbits]] |date=April 22, 1991 |access-date=September 21, 2009 |archive-date=June 8, 2011 |archive-url=https://web.archive.org/web/20110608031913/http://db.tidbits.com/series/1193 |url-status=live }}</ref> [[MODE32]] (originally published by [[Connectix]], and later licensed by Apple) contains a workaround that allows for larger SIMMs to be installed in Bank B if a PMMU is also installed. With this configuration, the Macintosh II boot ROMs will believe that the computer has 8 MB or less of RAM installed. Meanwhile, MODE32 then reprograms the memory controller on the fly to dedicate more address space to Bank A, thus allowing access to the additional memory installed in Bank B. Since this makes the physical address space discontiguous, the PMMU is then used to remap the address space into a contiguous block.<ref name=":1" />