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,
Commodore announced the Amiga 500 at the January 1987 winter Consumer Electronics Showat the same time as the high-end Amiga 2000. It was initially available in the Netherlands in April 1987, then the rest of Europe in May. In North America and the UK it was released in October 1987 with a list price. It competed directly against models in the Atari ST line.
The Amiga 500 was sold in the same retail outlets as the Commodore 64, as opposed to the computer store-only Amiga 1000. It proved to be Commodore's best-selling model, particularly in Europe. Although popular with hobbyists, arguably its most widespread use was as a gaming machine, where its graphics and sound were of significant benefit.
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. Also included was the Amiga video connector which allows the A500 to be used with a conventional CRT television.
In late 1991, the enhanced Amiga 500 Plus replaced the 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'.
The Amiga 500 series was discontinued in June 1992 and replaced by the similar Amiga 600. In late 1992, Commodore released the Amiga 1200, a machine closer in concept to the original Amiga 500, but with significant technical improvements. Despite this, neither the A1200 nor the A600 replicated the commercial success of its predecessor. By this time, the home market was strongly shifting to IBM PC compatibles with VGA graphics and the "low-cost" Macintosh Classic, LC, and IIsi models.
Description.
Outwardly resembling the Commodore 128 and codenamed "Rock Lobster" during development, the Amiga 500 houses the keyboard and CPU in one shell, unlike the Amiga 1000. It uses a Motorola 68000 microprocessor running at (NTSC) or (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. 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.) 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. 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 . 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 . There's a common trick to output sound with 14-bit precision that can be combined to output 14-bit sound.
The stock system comes with AmigaOS version 1.2 or 1.3 and of chip RAM (150 ns access time), one built-in double-density standard floppy disk drive that is completely programmable and can read IBM PC disks, standard Amiga disks, and up to 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 a DB23F connector. The then-standard RS-232 serial port (DB25M) and Centronics parallel port (DB25F) are also included. The power supply is (, ). 
The system displays video in analog RGB PAL or NTSC through a proprietary DB23M connector and in NTSC mode the line frequency is 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). On the left side, behind a plastic cover, there is a Zorro (Zorro I) bus expansion external edge connector with 86 pins. 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 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 ). 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 chip RAM by fitting the 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.
Technical specifications.
Graphics.
Max 6 bpp. The Amiga can show multiple resolution modes at the same time, splitting the screen vertically. An additional mode called Hold-And-Modify (HAM) makes it possible to use over a 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.
Memory.
Using various expansion techniques, the A500's total RAM can reach up to 138 MB – 2 MB Chip RAM, 8 MB 16-bit Fast RAM, and 128 MB 32-bit Fast RAM.
Chip RAM.
The stock 512 KB Chip RAM can be complemented by 512 KB using a "trapdoor" expansion (Commodore A501 or compatible). While that expansion memory is connected the chip bus, hardware limitations of the stock Agnus chip prevent its use as Chip RAM, only the CPU can access it. Suffering from the same contention limitations as Chip RAM, that memory is known as "Slow RAM" or "Pseudo-fast RAM". Retrofitting a later ECS Agnus and changing the memory-layout jumper allows use of trapdoor RAM as real Chip RAM for a total 1 MB.
Additionally, several third-party expansions exist with up to 1.8 MB on the trapdoor board. Using a Gary adapter, that memory is mapped similar to the standard Slow RAM and only usable by the CPU, yet slowed by chipset access.
Furthermore, using an A3000 Agnus on an adapter board, it is possible to expand the Chip RAM to 2 MB.
Fast RAM.
"Fast" RAM is located on the CPU-side bus. Its access is exclusive to the CPU and not slowed by any chipset access. The side expansion port allows for up to 8 MB of Zorro-style expansion RAM. Alternatively, a CPU adapter allows for internal expansion.
Accelerator RAM.
Internal or external CPU accelerators often include their own expansion memory. 16-bit CPUs are limited by the 24-bit address space but they can repurpose otherwise unused memory space for their included RAM. 32-bit CPU accelerators aren't limited by 24-bit addressing and can include up to 128 MB of Fast RAM (and potentially more).
Diagnostics.
When the computer is powered on a self-diagnostic test is run that will indicates failure with a specific colour:
The keyboard LED uses blink codes:
Trap-door expansion 501.
A popular expansion for the Amiga 500 was the Amiga 501 circuit board that can be installed underneath the computer behind a plastic cover. The expansion contains RAM configured by default as "Slow RAM" or "trap-door RAM" and a battery-backed real-time clock (RTC). The 512 KB trap-door RAM and 512 KB of original chip RAM will result in 1 MB of total memory. By default, the expansion memory is handled and reported by the system as fast memory. However, the RAM is physically connected to Agnus like chip RAM and it is impacted by chip-bus bandwidth contention. Being only accessible by the CPU but as slow as chip RAM, it is commonly being referred to as "Slow 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.