Amiga 500

The Amiga 500, also known as the A500 (or its code name "Rock Lobster"), 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 (730 USD 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 just above 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. Amiga 500 eventually sold 6 million units worldwide.^[4]

Releases

In October 1989, the Amiga 500 dropped its price from £499 GBP to £399 and was bundled with the Batman Pack in the United Kingdom. This price drop helped Commodore to sell more than 1 million Amiga 500s in 1989.^[5]

In late 1991, an enhanced model known as the Amiga 500+ replaced the original 500 in some markets, it was bundled with the Cartoon Classics pack in the United Kingdom at 399 GBP.^[6]

The Amiga 500 series was discontinued in mid-1992 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.^[7]

Description

Outwardly resembling the Commodore 128,^[8] 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 is 32-bit internally, but uses a 16-bit 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.^[9] 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 640×200 (up to 16 colors) for NTSC (704×484 overscan) and 320×256 to 640×256 for PAL (704×576 overscan.)^[10] 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.^[11] 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.^[12]

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 sockets 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.^[13] The then-standard RS-232 serial port (DB25M) and Centronics parallel port (DB25F) are also included. The power supply is (+5V, +/-12V).^[14] 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 II 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 “fast RAM" can be added using the side expansion slot.

The Amiga 500 has a "trap-door" slot on the underside for an upgrade of 512 KiB of RAM. The extra RAM is classified as "fast" RAM, but is sometimes referred to as "slow" RAM since 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 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 in the CPU socket. (Though the latter required 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    Green means no chip RAM found or is damaged,    Red means bad kickstart-ROM,    Yellow means mostly a bad CPU (no trap routine) or a bad Zorro expansion card.^[15]    Blue means custom chip problem (Denise, Paula, or Agnus),    Light Green means CIA problem,    Light Grey means (if it stops at Grey) that the CIA might be defective,     Black/stripes means ROM or CIA problem,    Black (no video) means really basic problem.^[16] 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.^[16] Using Caps Lock key and getting a response means CIA and the CPU works.^[16]

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.^[17] 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 KiB 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.

Graphics

• PAL mode: 320×256, 640×256, 640×512 (interlace),^[18] 704×576 in overscan.
• NTSC mode: 320×200, 640×200, 640×400 (interlace),^[18] 704×484 in overscan.

Max 6 bpp. The Amiga could show multiple resolution modes at the same time, splitting the screen vertically. An additional mode called Hold-And-Modify (HAM) makes it possible to utilize 12 bpp over a 3 pixels wide span. This works by letting each pixel position use the previous RGB value and modify one of the red, green or blue values to a new 4-bit value. This will cause some negligible colour artifacts however.

Connectors

Backside with connectors

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

• Two DE9M sockets for joysticks or mice (as popularized by the Atari 2600)
• 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^[13]
• A standard RS-232 serial port (DB25M)
• A parallel port (DB25F)
• Power inlet (+5 V, +/-12 V)^[14]
• 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

Expansions

• Expansion ports are limited to a side expansion port and a trapdoor expansion on the underside of the machine. The casing can also be opened up (voiding the warranty), all larger chips are socketed rather than being TH/SMD soldered directly to the motherboard, so they can be replaced by hand.
• The CPU can be upgraded to a Motorola 68010 directly or to a 68020, 68030 or 68040 via the side expansion slot or a CPU socket adapter board.
• The chip RAM can be upgraded to 1 MB directly on the motherboard, provided a Fat Agnus chip is also installed to support it.
• Likewise, all the custom chips can be upgraded to the ECS chipset.
• The A500+ model instead allowed upgrading by 1 MB trapdoor chip RAM without clock, but there was no visible means on board to map any of this as FAST, causing incompatibility with some stubbornly coded programs.
• There were modification instructions available for the A500 to solder or socket another 512 RAM on the board, then run extra address lines to the trapdoor slot to accommodate an additional 1 MB of fast or chip RAM depending on the installed chipset.
• Up to 8 MB of “fast RAM” can be added via the side expansion slot, even more if an accelerator with a non-EC (without reduced data/address bus) processor and 32-bit RAM is used.
• Hard drive and other peripherals can be added via the side expansion slot.
• Several companies provided combined CPU, memory and hard drive upgrades - or provided chainable expansions that extended the bus as they were added - as there is only one side expansion slot.
• Expansions are configured automatically by AutoConfig software, so multiple pieces of hardware did not conflict with each other.

Memory map

^[19]

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

See also

• Amiga models and variants
• Minimig - FPGA implementation

References

Wikimedia Commons has media related to Amiga 500.