Amiga 500

Commodore Amiga 500

An Amiga 500 computer system, with 1084S RGB monitor and second A1010 floppy disk drive
Type	Home computer
Release date	April 1987 (Netherlands), October 1987 (US)
Introductory price	US$699, £499 (1987) US$1,500 (2018 equivalent)
Discontinued	1991 (1991)
Media	880 KiB floppy disks
Operating system	AmigaOS v1.2 – 1.3
CPU	Motorola 68000 @ 7.16 MHz (NTSC) 7.09 MHz (PAL)
Memory	512 KiB 150 ns (9 MiB maximum[1])
Graphics	736×567i 4-bpp PAL (736×483i 4 bpp NTSC), 368×567i 6 bpp PAL (368×483i 6 bpp NTSC)
Sound	4× 8-bit channels PCM at max. 28 kHz with 6-bit volume in stereo
Predecessor	Amiga 1000
Successor	Amiga 500 Plus

The Amiga 500, also known as the A500, is 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. Before Amiga 500 was shipped, Commodore suggested that the list price of the Amiga 500 was US$595.95 without a monitor. At delivery in October 1987, Commodore announced that the Amiga 500 would carry a US$699/£499 list price. In Europe, the Amiga 500 was released in May 1987. In the Netherlands, the A500 was available from April 1987 for a list price of 1499 HFL (US$730 in 1987).^[2]

The Amiga 500 represents a return to Commodore's roots by being sold in the same mass retail outlets as the Commodore 64 – to which it was a spiritual successor – as opposed to the computer-store-only Amiga 1000, as well as being another computer whose keyboard is included in the same case.

The original Amiga 500 proved to be Commodore's best-selling Amiga model, enjoying particular success in Europe.^[3] Although popular with hobbyists, arguably its most widespread use was as a gaming machine, where its advanced graphics and sound were of significant benefit.

It has been claimed that over 6 million A500s were sold worldwide, however, according to Commodore UK, the entire sales of all Amigas in both Europe and the USA were 4-5 million. While not the first computer to have an open architecture, the Amiga is considered due to its expandability as one of the early examples.^[4]

Releases

In October 1989, the Amiga 500 dropped its price from £499 to £399 and was bundled with the Batman Pack in the United Kingdom (from October 1989 to September 1990) which included the games Batman, F/A-18 Interceptor, The New Zealand Story and the bitmap graphics editor, Deluxe Paint 2.^[5] Also included was the Commodore A520 RF Modulator, an adaptor which allowed the A500 to be used with a conventional CRT television set, via its RF antenna socket.

In late 1991, an enhanced model known as the Amiga 500 Plus replaced the original 500 in some markets; it was bundled with the Cartoon Classics pack in the United Kingdom at £399, although many stores still advertised it as an 'A500'.^[6]

The Amiga 500 series was discontinued in June 1992^[7] and replaced by the similarly specified and priced Amiga 600, although this new machine had originally been intended as a much cheaper model, which would have been the A300. In late 1992, Commodore released the "next-generation" Amiga 1200, a machine closer in concept to the original Amiga 500, but featuring significant technical improvements. Despite this, neither the A1200 nor the A600 replicated the commercial success of its predecessor, as by this time, the popular market was definitively shifting from the home computer platforms of the past to commodity Wintel PCs and the new "low-cost" Macintosh Classic, LC and IIsi models.^[8]

Description

Outwardly resembling the Commodore 128^[9] and codenamed "Rock Lobster" during development, the Amiga 500 houses the keyboard and CPU in one shell, unlike the Amiga 1000. It utilizes a Motorola 68000 microprocessor running at 7.15909 MHz (NTSC) or 7.09379 MHz (PAL). The CPU implements a 32-bit model, has 32-bit registers and 32-bit internal data bus, but it has a 16-bit main ALU, uses a 16-bit external data bus and 24-bit address bus, providing a maximum of 16 MB of address space.

The earliest Amiga 500 models use nearly the same Original Amiga chipset as the Amiga 1000.^[10] So graphics can be displayed in multiple resolutions and color depths, even on the same screen. Resolutions vary from 320×200 (up to 32 colors) to 640×400 (up to 16 colors) for NTSC (704×484 overscan) and 320×256 to 640×512 for PAL (704×576 overscan.)^[11] The system uses planar graphics, with up to five bitplanes (four in high resolution) allowing 2-, 4-, 8-, 16-, and 32-color screens, from a palette of 4096 colors. Two special graphics modes are also available: Extra HalfBrite, which uses a 6th bitplane as a mask to cut the brightness of any pixel in half (resulting in 32 arbitrary colors plus 32 more colors set at half the value of the first 32), and Hold And Modify (HAM) which allows all 4096 colors to be used on screen simultaneously.^[12] Later revisions of the chipset are PAL/NTSC switchable in software.

The sound chip produces four hardware-mixed channels, two to the left and two to the right, of 8-bit PCM at a sampling frequency of up to 28 kHz. Each hardware channel has its own independent volume level and sampling rate, and can be designated to another channel where it can modulate both volume and frequency using its own output. With DMA disabled it's possible to output with a sampling frequency up to 56 kHz. There's a common trick to output sound with 14-bit precision that can be combined to output 14-bit 56 kHz sound.^[13]

The stock system comes with AmigaOS version 1.2 or 1.3 and 512 KiB of chip RAM (150 ns access time), one built-in double-density standard floppy disk drive that is completely programmable and can read 720 KiB IBM PC disks, 880 KiB standard Amiga disks, and up to 984 KiB using custom-formatting drivers.

Despite the lack of Amiga 2000-compatible internal expansion slots, there are many ports and expansion options. There are two DE9M Atari joystick ports for joysticks or mice, stereo audio (RCA connectors 1 V p-p). There is a floppy drive port for daisy-chaining up to three extra floppy disk drives via an DB23F connector.^[14] The then-standard RS-232 serial port (DB25M) and Centronics parallel port (DB25F) are also included. The power supply is (+5V, +/-12V).^[15] The system displays video in analog RGB 50 Hz PAL or 60 Hz NTSC through a proprietary DB23M connector and in NTSC mode the line frequency is 15,750 Hz HSync for standard video modes, which is compatible with NTSC television and CVBS/RGB video, but out of range for most VGA-compatible monitors, while a multisync monitor is required for some of the higher resolutions. This connection can also be genlocked to an external video signal. The system was bundled with an RF adapter to provide output on televisions with a coaxial RF input, while monochrome composite video is available via an RCA connector (also coaxial). There is also a Zorro bus expansion on the left side (behind a plastic cover). Peripherals such as a hard disk drive can be added via the expansion slot and are configured automatically by the Amiga's AutoConfig standard, so that multiple devices do not conflict with each other. Up to 8 MB of so-called "fast RAM" (memory that can be accessed by the CPU only) can be added using the side expansion slot. This connector is electronically identical with the Amiga 1000's, but swapped on the other side.

The Amiga 500 has a "trap-door" slot on the underside for a RAM upgrade (typically of 512 KiB). This extra RAM is classified as "fast" RAM, but is sometimes referred to as "slow" RAM: due to the design of the expansion bus, it is actually on the chipset bus. Such upgrades usually include a battery-backed real-time clock. All versions of the A500 can have the additional RAM configured as chip RAM by a simple hardware modification, which involves fitting a later model (8372A) Agnus chip. Likewise, all versions of the A500 can be upgraded to 2 MB chip RAM by fitting the 8372B Agnus chip and adding additional memory.

The Amiga 500 also sports an unusual feature for a budget machine, socketed chips, which allow easy replacement of defective chips. The CPU can be directly upgraded on the motherboard to a 68010; or to a 68020, 68030, or 68040 via the side expansion slot; or by removing the CPU and plugging a CPU expansion card into the CPU socket (though this will require opening the computer and voiding any remaining warranty). In fact, all the custom chips can be upgraded to the Amiga Enhanced Chip Set (ECS) versions.

The case is made from ABS plastics which may become brown with time. This may be reversed by using the public-domain chemical mix "Retr0bright", though without a clearcoat to block oxygen, the brown colouring will return.

Whenever the computer is powered on a self-diagnostic test is run that will show any failure with a specific colour where    medium green means no chip RAM found or is damaged,    red means bad kickstart-ROM,    yellow means the CPU has crashed (no trap routine or trying to run bad code) or a bad Zorro expansion card.^[16]    Blue means custom chip problem (Denise, Paula, or Agnus),    light green means CIA problem. Stopping on    light gray means that the CIA might be defective.     Black and white stripes mean there is a ROM or CIA problem, while    black only (no video) means there is no video output.^[17] The keyboard LED uses blink codes: one blink means the keyboard ROM has a checksum error, two blinks means RAM failure, three blinks means watchdog timer failure.^[17] Using Caps Lock key and getting a response means CIA and the CPU works.^[17] There is no guarantee that every blinking state or color is accurate. Remember when the diagnostic codes are triggered it means the computer has some kind of fault and it can easily mis-interpret the fault and give false readings. For example, if the screen flashes green it can mean the Agnus is bad, the Agnus socket is bad, the logic connected to the Agnus is bad, the logic connected to the CPU is bad, the logic connected to the RAM is bad, a connection between CPU and/or logic and/or Agnus and/or Chip RAM is bad and/or some/all of the chip RAM is faulty etc. etc. Many of the issues with Amigas are caused by damage from corrosion or poor repair skills, especially the A500+ which has a Ni-Cad battery fitted and is always corroded if the battery has not been removed. Likewise a corroded battery on an A501 can cause faults on the A500 motherboard if the corrosion is very bad and has spread to the motherboard. The self-test chip RAM check is very brief and simplistic and all the other tests are minimalistic to minimize the start-up time (as documented in the Amiga Hardware Manual and many other official Commodore technical documents) so there is no guarantee that any of the diagnostic colours are 100% accurate.^[18]

Technical specifications

The standard Amiga 500 requires floppies to boot

• OCS (1.2 & 1.3 models) or ECS (1.3 and 500+ 2.04 models) chipset. ECS revisions of the chipset made PAL/NTSC mode switchable in software.
  • Graphics can be of arbitrary dimensions, resolution and colour depth, even on the same screen.
  • Without using overscan, the graphics can be 320 or 640 pixels wide by 200/256 or 400/512 pixels tall.
  • Planar graphics are used, with up to 5 bitplanes (4 in hires); this allowed 2, 4, 8, 16 and 32 colour screens, from a palette of 4096 colours. Two special graphics modes were also included: Extra HalfBrite, which used a 6th bitplane as a mask that halved the brightness of any colour seen, and Hold And Modify (HAM), which allowed all 4096 colours on screen at once.
  • Rhett Anderson developed the so-called Sliced HAM or SHAM mode, which was based on HAM mode, but used the Copper (part of the Agnus chip which could change hardware registers at given screen positions) to reprogram the color palette registers at each scanline. The advantage of SHM files was the ability to display all 4096 colors while eliminating the color blur of HAM compression.
  • Sound is 4 hardware-mixed channels of 8-bit sound at up to 28 kHz.^[19] The hardware channels have independent volumes (65 levels) and sampling rates, and are mixed down to two fully left and fully right stereo outputs. A software controllable low-pass audio filter is also included.
• 512 KB of chip RAM (150 ns access time).
• AmigaOS 1.2 or 1.3
• One double-density floppy disk drive is built in, which is completely programmable and thus can read 720 KiB IBM PC disks, 880 KiB standard Amiga disks, and up to 984 KiB with custom formatting (such as Klaus Deppich's diskspare.device). Uses 300 rpm (5 rotations/second) and 250 kbit/s.
• Built in keyboard.
• A two-button mouse is included.

Connectors

Backside with connectors

The Amiga 520 adapter allowed for an RF modulated output, to be connected to a TV

• Two DE9M Atari joystick ports for joysticks or mice
• Stereo audio (RCA connectors 1 V p-p)
• A floppy drive port (DB23F), for daisy-chaining up to 3 extra floppy disk drives via an DB23F connector^[14]
• A standard RS-232 serial port (DB25M)
• A parallel port (DB25F)
• Power inlet (+5 V, +/-12 V)^[15]
• Analogue RGB 50 Hz PAL and 60 Hz NTSC video output, provided on an Amiga-specific DB23M video connector. Can drive video with 15,750 Hz HSync for standard Amiga video modes. This is not compatible with most VGA monitors. A Multisync monitor is required for some higher resolutions. This connection can also be genlocked to an external video signal. An RF adapter (A520) was frequently bundled with the machine to provide output on regular televisions or on composite monitors. A digital 16 colour Red-Green-Blue-Intensity signal is available too on the same connector.
• Monochrome video via an RCA connector
• Zorro II bus expansion on the left side behind a plastic cover
• Trapdoor slot under the machine, for RAM expansion and real-time clock

Memory map

^[21]

Trap-door expansion 501

An A501 compatible expansion

A popular expansion for the Amiga 500 was the Amiga 501 circuit board that can be installed underneath the computer behind a plastic cover. It contains 512 KiB RAM configured by default as "Slow RAM" or "trap-door RAM" and a battery-backed real-time clock (RTC). However, the RAM is pseudo-fast RAM, only accessible by the processor, but still as slow as chip RAM. The motherboard can be modified to relocate the trap-door RAM to the chip memory pool, provided a compatible Agnus chip is fitted on the motherboard.

Notable uses

• Digital hardcore group EC8OR recorded their premiere album using only an Amiga 500 and a microphone.
• Skam group Team Doyobi recorded their premiere album Cryptoburners using only an Amiga 500 and no microphone.
• Aggrotech artist X-fusion also recorded music from 1989 to 1992 using an Amiga 500 (and occasionally Kawai K4 and/or Roland JD-800).

See also

• Amiga models and variants
• Minimig – an FPGA implementation

References

Wikimedia Commons has media related to Amiga 500.