Chip RAM
Encyclopedia
Chip RAM is a commonly used term for the integrated RAM used in Commodore
's line of Amiga
computers. Chip RAM is shared between the central processing unit
(CPU) and the Amiga's dedicated chipset (hence the name). It was also, rather misleadingly, known as "graphics RAM".
models) coprocessor is the direct memory access
(DMA) controller. 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 the first Amiga systems can only access memory on every second clock cycle, Agnus was designed to operate a system where the time-critical custom chip DMA gets the "odd" clock cycle and the CPU gets the "even" cycle, thus the CPU is not blocked from memory access and runs without interruption. However, certain chipset DMA, such as copper or blitter
operations, can use any spare cycles, effectively blocking even cycles from the CPU. In such situations CPU cycles are only blocked while accessing shared RAM, but never when accessing external RAM or ROM.
The shared RAM data bus is 16-bit on OCS and ECS systems. 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 bandwidth
is increased fourfold compared to the earlier 16-bit design. The ECS-based A3000 also has 32-bit shared 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 DIP package able to address 512 kiB of chip RAM. Subsequent versions of the Agnus are in an 84-pin PLCC package (either socketed or surface-mounted
). All models, except the A1000, are upgradable to 2 MiB of chip RAM. The A500, and later versions of the A2000, with hardware modification can accommodate 1 MiB by installing a later revision Agnus chip (8732A). Likewise, 2 MB can be installed by fitting an 8372B Agnus and extra memory.
The maximum amount of chip RAM in any model is 2 MiB. The Amiga 4000 motherboard includes a non-functional jumper that claims to accommodate 8 MiB of chip RAM - regardless of its position, the system only recognizes 2 MiB due to the limitations of the Alice chip. However, the software emulator, UAE, can emulate an Amiga system with up to 8 MiB of chip RAM.
.
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 expanded Amiga 500 contain 512 KB pseudo fast RAM ("slow RAM") controlled by Agnus with the same limitations as chip RAM, yet unusable as such due to register limitations. Numerous budget trapdoor expansions for the 500 extended this 'controllerless' concept to up to 1.8 MB 'slow' RAM (requiring a Gary adapter for addressing).
Commodore International
Commodore is the commonly used name for Commodore Business Machines , the U.S.-based home computer manufacturer and electronics manufacturer headquartered in West Chester, Pennsylvania, which also housed Commodore's corporate parent company, Commodore International Limited...
's line of Amiga
Amiga
The Amiga is a family of personal computers that was sold by Commodore in the 1980s and 1990s. The first model was launched in 1985 as a high-end home computer and became popular for its graphical, audio and multi-tasking abilities...
computers. Chip RAM is shared between the central processing unit
Central processing unit
The central processing unit is the portion of a computer system that carries out the instructions of a computer program, to perform the basic arithmetical, logical, and input/output operations of the system. The CPU plays a role somewhat analogous to the brain in the computer. The term has been in...
(CPU) and the Amiga's dedicated chipset (hence the name). It was also, rather misleadingly, known as "graphics RAM".
Direct memory access
Under the Amiga architecture, the Agnus (Alice on AGAAGA
-Medicine:* Adequate, Appropriate or Average for Gestational Age, referring to prenatal growth rate* Anti-gliadin antibodies* Androgenetic alopecia* Aspartylglucosaminidase-Business:* AGA , Autogen Gasaccumulator AG, 1920's German car company...
models) coprocessor is the direct memory access
Direct memory access
Direct memory access is a feature of modern computers that allows certain hardware subsystems within the computer to access system memory independently of the central processing unit ....
(DMA) controller. 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 the first Amiga systems can only access memory on every second clock cycle, Agnus was designed to operate a system where the time-critical custom chip DMA gets the "odd" clock cycle and the CPU gets the "even" cycle, thus the CPU is not blocked from memory access and runs without interruption. However, certain chipset DMA, such as copper or blitter
Blitter
In a computer system, a blitter is a circuit, sometimes as a coprocessor or a logic block on a microprocessor, that is dedicated to the rapid movement and modification of data within that computer's memory...
operations, can use any spare cycles, effectively blocking even cycles from the CPU. In such situations CPU cycles are only blocked while accessing shared RAM, but never when accessing external RAM or ROM.
Chip RAM by model
Most stock Amiga systems were equipped with chip RAM only and shipped with between 256 kB and 2 MB.| Model | Stock chip RAM | Maximum chip RAM | Width |
|---|---|---|---|
| Amiga 1000 Amiga 1000 The A1000, or Commodore Amiga 1000, was Commodore's initial Amiga personal computer, introduced on July 23, 1985 at the Lincoln Center in New York City.... |
256 kiB | 512 kiB | 16-bit |
| Amiga 500 Amiga 500 The Amiga 500 - also known as the A500 - was the first “low-end” Commodore Amiga 16/32-bit multimedia home/personal computer. It was announced at the winter Consumer Electronics Show in January 1987 - at the same time as the high-end Amiga 2000 - and competed directly against the Atari 520ST... , Amiga 2000 Amiga 2000 The Amiga 2000, or A2000, is a personal computer released by Commodore in 1986. It is the successor to the Amiga 1000.-Features:Aimed at the high-end market, the original Europe-only model adds a Zorro II backplane, implemented in programmable logic, to the custom Amiga chipset used in the Amiga 1000... |
512 kiB - 1 MiB | 512 kiB - 1 MiB | 16-bit |
| CDTV CDTV The CDTV was a multimedia platform developed by Commodore International and launched in 1991. On a technological level it was essentially a Commodore Amiga 500 home computer in a Hi-Fi style case with a single-speed CD-ROM drive. Commodore marketed the machine as an all-in-one home multimedia... , Amiga 500 Plus, Amiga 600 Amiga 600 The Amiga 600, also known as the A600 , is a home computer that was introduced at the CeBIT show in March 1992. The A600 was Commodore International's final model based on the Motorola 68000 CPU and the ECS chipset. It is essentially a redesign of the Amiga 500 Plus, with the option of an internal... |
1 MiB | 2 MiB | 16-bit |
| Amiga 3000 Amiga 3000 The Commodore Amiga 3000, or A3000, was the third major release in the Amiga computer family. Released in June 1990, it features improved processing speed, improved rendering of graphics, and a new revision of the operating system... |
1 MiB | 2 MiB | 32-bit |
| Amiga 1200 Amiga 1200 The Amiga 1200, or A1200 , was Commodore International's third-generation Amiga computer, aimed at the home market... , Amiga 4000 Amiga 4000 The Commodore Amiga 4000, or A4000, is the successor of the A2000 and A3000 computers. There are two models, the A4000/040 released in October 1992 with a Motorola 68040 CPU, and the A4000/030 released in April 1993 with a Motorola 68EC030.... , Amiga CD32 Amiga CD32 The Amiga CD32, styled "CD32" , was the first 32-bit CD-ROM based video game console released in western Europe, Australia, Canada and Brazil. It was first announced at the Science Museum in London, United Kingdom on 16 July 1993, and was released in September of the same year... |
2 MiB | 2 MiB | 32-bit |
The shared RAM data bus is 16-bit on OCS and ECS systems. 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 bandwidth
Memory bandwidth
Memory bandwidth is the rate at which data can be read from or stored into a semiconductor memory by a processor. Memory bandwidth is usually expressed in units of bytes/second, though this can vary for systems with natural data sizes that are not a multiple of the commonly used 8-bit bytes.Memory...
is increased fourfold compared to the earlier 16-bit design. The ECS-based A3000 also has 32-bit shared 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 DIP package able to address 512 kiB of chip RAM. Subsequent versions of the Agnus are in an 84-pin PLCC package (either socketed or surface-mounted
Surface-mount technology
Surface mount technology is a method for constructing electronic circuits in which the components are mounted directly onto the surface of printed circuit boards . An electronic device so made is called a surface mount device...
). All models, except the A1000, are upgradable to 2 MiB of chip RAM. The A500, and later versions of the A2000, with hardware modification can accommodate 1 MiB by installing a later revision Agnus chip (8732A). Likewise, 2 MB can be installed by fitting an 8372B Agnus and extra memory.
The maximum amount of chip RAM in any model is 2 MiB. The Amiga 4000 motherboard includes a non-functional jumper that claims to accommodate 8 MiB of chip RAM - regardless of its position, the system only recognizes 2 MiB due to the limitations of the Alice chip. However, the software emulator, UAE, can emulate an Amiga system with up to 8 MiB of chip RAM.
Fast RAM
Amiga systems can also be expanded with, so called, "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 approximately doubles the instruction speedInstructions per second
Instructions per second is a measure of a computer's processor speed. Many reported IPS values have represented "peak" execution rates on artificial instruction sequences with few branches, whereas realistic workloads typically lead to significantly lower IPS values...
.
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 expanded Amiga 500 contain 512 KB pseudo fast RAM ("slow RAM") controlled by Agnus with the same limitations as chip RAM, yet unusable as such due to register limitations. Numerous budget trapdoor expansions for the 500 extended this 'controllerless' concept to up to 1.8 MB 'slow' RAM (requiring a Gary adapter for addressing).