View source for Amiga Chip RAM

'''Chip RAM'''<ref name="auisg_ref">{{cite book|author=Commodore-Amiga, Inc.|date=1991|title=Amiga User Interface Style Guide|series=Amiga Technical Reference Series|publisher=Addison-Wesley Publishing Company, Inc.|location=Reading, Massachusetts|page=180|isbn=978-0-201-57757-0}}</ref> is a commonly used term for the integrated [[random-access memory|RAM]] used in [[Commodore International|Commodore]]'s line of [[Amiga]] computers. Chip RAM is shared between the [[central processing unit]] (CPU) and the Amiga's dedicated [[Amiga Original chipset|chipset]] (hence the name).<ref name="amiga_hardware_ref">{{cite book|author=Commodore-Amiga, Inc.|title=Amiga Hardware Reference Manual|edition=Third|series=Amiga Technical Reference Series|year=1991|publisher=Addison-Wesley Publishing Company, Inc.|location=Reading, Massachusetts|isbn=978-0-201-56776-2}}</ref> It was also, rather misleadingly, known as "graphics RAM".

== Direct memory access ==
Under the Amiga architecture, the [[direct memory access]] (DMA) controller is integrated into the [[MOS Technology Agnus|Agnus]] (Alice on [[Amiga Advanced Graphics Architecture|AGA]] models) chip.  Both the CPU and other members of the chipset have to arbitrate for access to shared RAM via Agnus.  This allows the custom chips to perform video, audio, or other DMA operations independently of the CPU.  As the [[68000]] processor used in early Amiga systems usually only accesses memory on every second memory cycle, Agnus operates a system where the "odd" clock cycle is allocated to time-critical custom chip access and the "even" cycle is allocated to the CPU: thus, for average DMA demand, the CPU is not typically blocked from memory access and may run without interruption.  However, certain chipset DMA, such as high-resolution graphics with a larger color palette, [[Original Amiga chipset#Copper|Copper]], or [[blitter]] operations, can use any spare cycles, effectively blocking cycles from the CPU.  In such situations CPU cycles are only blocked while accessing shared RAM, but never when accessing Fast (CPU-only) RAM (when present) or [[read-only memory|ROM]].<ref name="amiga_hardware_ref"/>

== Chip RAM by model ==
Most stock Amiga systems were equipped with Chip RAM only and shipped with between 256&nbsp;[[binary prefix|kiB]] and 2&nbsp;MiB.

{| class="wikitable"
! Model
! Stock Chip RAM
! Maximum Chip RAM
! Width
|-
|[[Amiga 1000]]
|256&nbsp;KiB
|512&nbsp;KiB
|16-bit
|-
|[[Amiga 500]], [[Amiga 2000]], [[CDTV]]
|512&nbsp;KiB &ndash; 1&nbsp;MiB{{ref label|note_a|a|^}}
|512&nbsp;KiB &ndash; 1&nbsp;MiB{{ref label|note_b|b|^}}
|16-bit
|-
|[[Amiga 500 Plus]], [[Amiga 600]]
|1&nbsp;MiB
|2&nbsp;MiB
|16-bit
|-
|[[Amiga 3000]]
|1&nbsp;MiB
|2&nbsp;MiB
|32-bit{{ref label|note_c|c|^}}
|-
|[[Amiga 1200]], [[Amiga 4000]], [[Amiga CD32]]
|2&nbsp;MiB
|2&nbsp;MiB
|32-bit{{ref label|note_d|d|^}}
|}

The shared RAM [[data bus]] is 16-bit on [[Original Chip Set|OCS]] and [[Amiga Enhanced Chip Set|ECS]] systems{{ref label|note_c|c|^}}.  The later AGA systems use a 32-bit data bus controlled by the Alice coprocessor (replacing Agnus) and 32-bit RAM.  The memory clock runs at double the rate on AGA systems.  As a result, chipset RAM [[memory bandwidth|bandwidth]] is increased fourfold compared to the earlier 16-bit design.  However, 32-bit access is limited to CPU and graphics DMA and cannot be used for other devices.  The ECS-based A3000 also has 32-bit Chip RAM, but access is only 32-bit for CPU operations; the chipset remained 16-bit.

The maximum amount of Chip RAM is dependent on the Agnus/Alice version.  The original Agnus chip fitted to the A1000 and early A2000 systems is a 48-pin [[Dual in-line package|DIP]] package able to address 512&nbsp;KiB of Chip RAM.  Subsequent versions of the Agnus are in an 84-pin [[Chip_carrier#Plastic-leaded_chip_carrier|PLCC]] package (either socketed or [[surface-mount technology|surface-mounted]]).  All models except the A1000 are upgradable to 2&nbsp;MiB of Chip RAM.  The A500 and the early A2000B can accommodate 1&nbsp;MiB by installing a later revision Agnus chip (8732A) with minimal hardware modifications; late-production machines usually already contained that chip, so that only [[Jumper (computing)|jumper]] modifications were necessary.  Likewise, 2&nbsp;MB can be installed by fitting an 8372B Agnus and extra memory.

The maximum amount of Chip RAM in any model is 2&nbsp;MiB.  The Amiga 4000 [[motherboard]] includes a non-functional jumper that anticipated later chips and is labeled for 8&nbsp;MiB of Chip RAM&mdash;regardless of its position, the system only recognizes 2&nbsp;MiB due to the limitations of the Alice chip.<ref>{{cite web | url=http://www.zimmers.net/cbmpics/cbm/amiga/a4000jumpers.html | title=A4000 Motherboard Jumpers | accessdate=2 September 2011 | archive-url=https://web.archive.org/web/20191123044708/http://www.zimmers.net/cbmpics/cbm/amiga/a4000jumpers.html | archive-date=23 November 2019 | url-status=dead }}</ref>  However, the software emulator [[UAE (emulator)|UAE]] can emulate an Amiga system with the design limit of up to 8&nbsp;MiB of Chip RAM.<ref name="uae">{{cite web | title=Features of the UAE Amiga Emulator | url=http://amiga.technology/features | accessdate=4 January 2017 | archive-url=https://web.archive.org/web/20170105180734/http://amiga.technology/features | archive-date=2017-01-05 | url-status=dead }}</ref>

== Fast RAM ==
Amiga systems can also be expanded with Fast RAM, which is only accessible to the CPU.  This improves execution speed, as CPU cycles are never blocked even when the custom chipset is simultaneously accessing Chip RAM.  Adding Fast RAM to systems with [[32-bit]] CPUs roughly doubles the [[instructions per second|instruction speed]], as the more advanced [[68020]], [[68030|'030]], and [[68040|'040]] CPUs can utilize more memory cycles than the earlier 68000.

Confusingly, a system may have several different kinds and speeds of Fast RAM.  For example, an Amiga 3000 may contain 16-bit [[Zorro II]] expansion RAM, 32-bit [[Zorro III]] expansion RAM, 32-bit motherboard RAM, and 32-bit CPU card RAM simultaneously (in increasing speed order).  Automatically configured RAM is prioritized by the system, so the fastest memory is used first.

Early versions of the Amiga 2000B, and the most common "trapdoor memory expanded" configuration of the Amiga 500, contain 512&nbsp;KiB pseudo-fast RAM ("slow RAM") controlled by Agnus with the same limitations as Chip RAM, yet unusable as such due to [[hardware register|register]] limitations.  Numerous budget trapdoor expansions<ref>{{cite web | url=http://amiga.resource.cx/search.pl?product=&company=&amiga=500&intf=trap&mid=&base=dec&pid= | title=List of A500 Trapdoor Expansions | accessdate=3 November 2018}}</ref> for the 500 extended this "controllerless" concept to up to 1.8&nbsp;MB<!-- what's the exact amount? --> slow RAM (requiring a [[Amiga custom chips#Gary|Gary]] adapter for addressing).

==Notes==
<ol type="a">
<li>{{note label|note_a|a|^}} Later A500/A2000 systems were equipped with 1&nbsp;MiB
<li>{{note label|note_b|b|^}} Up to 2&nbsp;MiB with hardware modification
<li>{{note label|note_c|c|^}} A3000 Chip RAM is 32-bit for CPU access, 16-bit for chipset access
<li>{{note label|note_d|d|^}} AGA Chip RAM access is 32-bit for CPU and graphics DMA only; audio, floppy, Copper, and blitter DMA remain 16-bit
</ol>

== References ==
{{reflist}}

== See also ==

{{Portal|Amiga}}

{{Amiga hardware}}

{{DEFAULTSORT:Amiga Chip RAM}}
[[Category:Amiga]]