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Maximum RAM Supported by Amiga Emulation

 

TOPIC

Q: How much RAM does the Amiga emulation support?

 

DISCUSSION

A: In general, the maximum memory of all Amiga systems mirrors the limitations of the original hardware. As some applications and system functions may fail when using more RAM than was possible on the original Amiga hardware (e.g. 2 MB Chip RAM or 128 MB of Zorro III RAM),  the original configurations are set to avoid unsafe settings.

In accordance with the original specifications, the Amiga Chip RAM is limited to 2 MB on newer Amiga systems (AGA chipset), and to 512 KB on early models (OCS chipset). The maximum amount of Fast RAM is always 8 MB. Systems with a 68020 or higher CPU further support up to 128 MB or more Zorro III RAM (which can be used instead of Fast RAM). Systems with expansion options further support up to 512 MB of RTG (retargetable graphics, or video) RAM, which is supported by the fully licensed Picasso96 software that is included with Amiga Forever.

The Amiga Autoconfig mechanism supports Zorro III RAM expansion boards with memory totals being an exact power of 2, between 64 KB and 1 GB. When a different value is set, Amiga Forever generates the correct number of boards to achieve the desired goal.

Originally, the address space available to Zorro III expansion boards was documented as being 1 GB (0x40000000 to 0x7FFFFFFF). This is shared by pure RAM expansions, RTG video memory, and other peripherals.

In practice, the total hardware memory space of the Amiga Zorro III expansion bus is 1792 MB. On A-4000 class systems (e.g. Workbench 3.X, Amiga 4000, AROS and WHDLoad configurations), the emulation can thus achieve a total of 1.5 GB Zorro III RAM by emulating two Zorro III expansion boards (1 GB + 512 MB), if peripherals like the RTG video memory leave room for it.

On the software side, PowerPC-based operating systems like AmigaOS 4.1 FE enforce the restriction imposed by the original documentation, ignoring any Zorro III boards (RAM or other types) which fall outside of the original 1 GB range. Additionally, because memory is virtualized, memory expansion boards need to be mapped into the heap (i.e. memory set aside for dynamic allocation), which is limited to 768 MB, including both Zorro III RAM expansion boards and other types of memory (Chip RAM, PowerPC RAM).

Older 68K versions of the operating system do not enforce the 1 GB limit, nor do they need to work with virtual addresses and related heap requirements. They therefore also support Zorro III boards (RAM and other types) in the address space upwards of 0x80000000.

The process address space of the host environment also has an impact on the maximum contiguous memory that can be allocated. On 32-bit versions of Windows the maximum size of Zorro III RAM expansions may be about 512 MB. On 64-bit versions of Windows, the practical limit when using 32-bit versions of the emulation software (which is the current default) may be about 1 GB or 1.5 GB.

In order to allocate the maximum Amiga address space, Amiga Forever (2016 and newer) must be set to use the 64-bit version of the emulation. This can be achieved by setting the Use 64-bit version option in the Emulation settings (under Tools/Options in Amiga Forever).

In all cases, since the host environment's virtual memory functionality is inherited, it is possible to emulate an Amiga having more RAM than is actually available on the computer in which the emulation is run.

 

Article Information
Article ID:13-111
Platform:Windows
Products:Amiga Forever
Additional Keywords:None
Last Update:2015-12-23
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