Amiga Chip RAM

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Chip RAM [1] 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). [2] It was also, rather misleadingly, known as "graphics RAM".

Contents

Direct memory access

Under the Amiga architecture, the direct memory access (DMA) controller is integrated into the Agnus (Alice on 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, 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 ROM. [2]

Chip RAM by model

Most stock Amiga systems were equipped with Chip RAM only and shipped with between 256  kiB and 2 MiB.

ModelStock Chip RAMMaximum Chip RAMWidth
Amiga 1000 256 KiB512 KiB16-bit
Amiga 500, Amiga 2000, CDTV 512 KiB – 1 MiB [a] 512 KiB – 1 MiB [b] 16-bit
Amiga 500 Plus, Amiga 600 1 MiB2 MiB16-bit
Amiga 3000 1 MiB2 MiB32-bit [c]
Amiga 1200, Amiga 4000, Amiga CD32 2 MiB2 MiB32-bit [d]

The shared RAM data bus is 16-bit on OCS and ECS systems [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 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 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 the early A2000B can accommodate 1 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 modifications were necessary. 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 anticipated later chips and is labeled for 8 MiB of Chip RAM—regardless of its position, the system only recognizes 2 MiB due to the limitations of the Alice chip. [3] However, the software emulator UAE can emulate an Amiga system with the design limit of up to 8 MiB of Chip RAM. [4]

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 instruction speed, as the more advanced 68020, '030, and '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 KiB 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 [5] for the 500 extended this "controllerless" concept to up to 1.8 MB slow RAM (requiring a Gary adapter for addressing).

Notes

  1. ^ Later A500/A2000 systems were equipped with 1 MiB
  2. ^ Up to 2 MiB with hardware modification
  3. ^ A3000 Chip RAM is 32-bit for CPU access, 16-bit for chipset access
  4. ^ AGA Chip RAM access is 32-bit for CPU and graphics DMA only; audio, floppy, Copper, and blitter DMA remain 16-bit

Related Research Articles

Amiga Family of personal computers sold by Commodore

The Amiga is a family of personal computers introduced by Commodore in 1985. The original model is one of a number of computers with 16 or 32-bit processors, 256 KB or more of RAM, mouse-based GUIs, and significantly improved graphics and audio over 8-bit systems. This includes the Atari ST—released earlier the same year—Macintosh, and the Acorn Archimedes. Based on the Motorola 68000 microprocessor, the Amiga differs from its contemporaries through the inclusion of custom hardware to accelerate graphics and sound, including sprites and a blitter, and a pre-emptive multitasking operating system called AmigaOS.

Amiga 500

The Amiga 500, also known as the A500, is the first low-end version of the Amiga home computer. It contains the same Motorola 68000 as the Amiga 1000, as well as the same graphics and sound coprocessors, but is in a smaller case similar to that of the Commodore 128,

Original Chip Set

The Original Chip Set (OCS) is a chipset used in the earliest Commodore Amiga computers and defined the Amiga's graphics and sound capabilities. It was succeeded by the slightly improved Enhanced Chip Set (ECS) and greatly improved Advanced Graphics Architecture (AGA).

Amiga 600

The Amiga 600, also known as the A600, is a home computer introduced in March 1992. It is the final Amiga model based on the Motorola 68000 and the 1990 Amiga Enhanced Chip Set. A redesign of the Amiga 500 Plus, it adds the option of an internal hard disk drive and a PCMCIA port. Lacking a numeric keypad, the A600 is only slightly larger than an IBM PC keyboard and weighing approximately 6 pounds. It shipped with AmigaOS 2.0, which was considered more user-friendly than earlier versions of the operating system.

Amiga 3000

The Commodore Amiga 3000, or A3000, is a personal computer released by Commodore in June 1990. It features improved processing speed, improved graphics rendering, and a new revision of the operating system. It is the successor to the Amiga 2000.

Amiga 2000

The Amiga 2000, or A2000, is a personal computer released by Commodore in March 1987. It was introduced as a "big box" expandable variant of the Amiga 1000 but quickly redesigned to share most of its electronic components with the contemporary Amiga 500 for cost reduction. Expansion capabilities include two 3.5" drive bays and one 5.25" bay that can be used by a 5.25" floppy drive, a hard drive, or CD-ROM once they became available.

Amiga Advanced Graphics Architecture (AGA) is the third-generation Amiga graphic chipset, first used in the Amiga 4000 in 1992. Before release AGA was codenamed Pandora by Commodore International.

A blitter is a circuit, sometimes as a coprocessor or a logic block on a microprocessor, dedicated to the rapid movement and modification of data within a computer's memory. A blitter can copy large quantities of data from one memory area to another relatively quickly, and in parallel with the CPU, while freeing up the CPU's more complex capabilities for other operations. A typical use for a blitter is the movement of a bitmap, such as windows and fonts in a graphical user interface or images and backgrounds in a 2D video game. The name comes from the bit blit operation of the 1973 Xerox Alto, which stands for bit-block transfer. A blit operation is more than a memory copy, because it can involve data that's not byte aligned, handling transparent pixels, and various ways of combining the source and destination data.

Amiga 4000 Commodore computer

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.

Chipset Set of electronic components in an integrated circuit

In a computer system, a chipset is a set of electronic components in an integrated circuit known as a "Data Flow Management System" that manages the data flow between the processor, memory and peripherals. It is usually found on the motherboard. Chipsets are usually designed to work with a specific family of microprocessors. Because it controls communications between the processor and external devices, the chipset plays a crucial role in determining system performance.

MOS Technology Agnus

The MOS Technology "Agnus", usually called Agnus, is an integrated circuit in the custom chipset of the Commodore Amiga computer. The Agnus, Denise and Paula chips collectively formed the OCS and ECS chipsets.

Hold-And-Modify

Hold-And-Modify, usually abbreviated as HAM, is a display mode of the Commodore Amiga computer. It uses a highly unusual technique to express the color of pixels, allowing many more colors to appear on screen than would otherwise be possible. HAM mode was commonly used to display digitized photographs or video frames, bitmap art and occasionally animation. At the time of the Amiga's launch in 1985, this near-photorealistic display was unprecedented for a home computer and it was widely used to demonstrate the Amiga's graphical capability. However, HAM has significant technical limitations which prevent it from being used as a general purpose display mode.

The Amiga is a family of home computers that were designed and sold by the Amiga Corporation from 1985 to 1994.

The AAA chipset was intended to be the next-generation Amiga multimedia system designed by Commodore International. Initially begun as a secret project, the first design discussions were started in 1988, and after many revisions and redesigns the first silicon versions were fabricated in 1992–1993. The project was all but abandoned in 1993 after it was projected that PCs were to equal the AAA shortly after release, so a further jump was needed, leading to project Hombre. AAA was not designed to be AGA compatible.

Minimig

Minimig is an open source re-implementation of an Amiga 500 using a field-programmable gate array (FPGA).

Amiga 4000T

The Amiga 4000T, also known as A4000T, is a tower version of Commodore's A4000 personal computer. Using the AGA chipset, it was originally released in small quantities in 1994 with a 25 MHz Motorola 68040 CPU, and re-released in greater numbers by Escom in 1995, after Commodore's demise, along with a new variant which featured a 50 MHz Motorola 68060 CPU. Despite the subsequent demise of Escom, production was continued by QuikPak in North America into at least 1997.

Hombre is a RISC chipset for the Amiga, designed by Commodore, which was intended as the basis of its next generation game machine called CD64 and a 3D accelerator PCI card. Hombre was canceled along with the bankruptcy of Commodore International.

Amiga 1200

The Amiga 1200, or A1200, is a personal computer in the Amiga computer family released by Commodore International, aimed at the home computer market. It was launched on October 21, 1992, at a base price of £399 in the United Kingdom and $599 in the United States.

This was the last classic Amiga compatible chipset that Commodore announced in 1992. They planned to release it in 1994 for low-end Amiga computers along with AAA.

In addition to the Amiga chipsets, various specially designed chips have been used in Commodore Amiga computers that do not belong to the 'Amiga chipset' in a tight sense.

References

  1. ↑ Commodore-Amiga, Inc. (1991). Amiga User Interface Style Guide. Amiga Technical Reference Series. Reading, Massachusetts: Addison-Wesley Publishing Company, Inc. p. 180. ISBN   978-0-201-57757-0.
  2. 1 2 Commodore-Amiga, Inc. (1991). Amiga Hardware Reference Manual. Amiga Technical Reference Series (Third ed.). Reading, Massachusetts: Addison-Wesley Publishing Company, Inc. ISBN   978-0-201-56776-2.
  3. ↑ "A4000 Motherboard Jumpers". Archived from the original on 23 November 2019. Retrieved 2 September 2011.CS1 maint: discouraged parameter (link)
  4. ↑ "Features of the UAE Amiga Emulator". Archived from the original on 2017-01-05. Retrieved 4 January 2017.CS1 maint: discouraged parameter (link)
  5. ↑ "List of A500 Trapdoor Expansions" . Retrieved 3 November 2018.CS1 maint: discouraged parameter (link)

See also

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