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Amstrad CPC

From Academic Kids

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Amstrad_CPC464.jpg
Amstrad CPC 464, with CTM644 colour monitor

The Amstrad CPC was an 8-bit home computer produced by Amstrad in the 1980s. CPC stood for 'Colour Personal Computer', although it was possible to purchase a CPC with a green screen (GT65/66) as well as with the standard colour screen (CTM640). The machine was designed to be a direct competitor to the Commodore 64 and Sinclair ZX Spectrum systems.

Outwardly, the most distinguishing features of Amstrad's offering were the matt black console case with sharp corners and narrowly rectangular form factor (the latter due to the built-in cassette tape deck (CPC 464) or floppy disk drive (CPC 664 and CPC 6128), the keyboard's distinctly coloured special keys (all the non-typewriter-standard keys), and the unique power supply hookup with one lead going from the console to the monitor (or RF modulator) and one lead going the other way. A television could be used with an optional adapter.

Contents

The Amstrad CPC sold as a "complete system"

Amstrad initially promoted the CPC as being an improvement on the competing ZX Spectrum and C64 because it was a complete system - including everything required to use the machine in one box. Compared to a C64 or a ZX Spectrum, the Amstrad CPCs shipped with their own monitor, had a built in tape recorder or floppy disk drive and even a small loudspeaker. This marketing gave a more "professional" appeal to the Amstrad CPC by marketing it in the same way as business-oriented systems, rather than gaming or home oriented ones.

As a late entrant to the European 8-bit market, the CPC range never achieved the sales volume of either the ZX Spectrum or the C64, but the advantages of a proper typewrite-style keyboard and integrated tape or floppy drive saw it obtain considerable market share in the late 80s.

The CPC Computer Family

The Amstrad CPC 464, 472, 664, 6128

The original CPC was sold in the following configurations:

  • CPC 464 – Tape Deck, 64K RAM, square-edged keyboard
  • CPC 472 – Tape Deck, 72KB RAM. Produced in small numbers for the Spanish market in the hope of avoiding a legal ruling requiring the production of a Spanish keyboard
  • CPC 664 – 3" Floppy Disk Drive, 64K RAM, bowed keyboard
  • CPC 6128 – 3" Floppy Disk Drive, 128K RAM (accessed using bank switching), more PC-style keyboard

External disk drives such as the DDI-1/FD-1 were available for the 464. A cassette adapter was available for the 664 and 6128. All models are overwhelmingly compatible with one another. A hardware addon, the Multiface allowed backup of most tape software to disc.

The CPC 664 proved short-lived, quickly replaced by the better-specified CPC 6128. The memory layout of the system allowed the CPC to run CP/M 2.2 and CP/M software adapted especially for the machine was not uncommon.

An Amstrad specific variant of CP/M known as CP/M 3.0 or CP/M Plus was developed for the 6128 and used in later years.

Most games and software were targeted the 64kb 464 and 664. Only a handful of titles exclusively targeted the 128kb machines.

West Germany: Schneider CPC 464, 664 and 6128

Amstrad's German partner company Schneider produced its own models of the CPC 464, 664 and 6128. These machines had grey keys in place of the Amstrad coloured alternatives and an industry standard Centronics port in place of the expansion edge connector but were otherwise identical at the hardware level. Documentation and case labels were translated into German.

East Germany: KC compact

Like most other computers of the era, the CPC inspired a clone in the Eastern bloc, the KC compact made in East Germany using Russian and East German components.

The machine differs from a CPC visually with a different style of case, external power supply and (optional and even more scarce than the main device) external 5.25" 'Robotron' disc drive. Unlike the Amstrad models it may be used with a televsion screen out of the box. It runs BASIC 1.1 and a CP/M 2.6 clone, the German speaking MicroDOS. It has 64K RAM built in and an additional 64K RAM is provided with the external disc/tape drive adapter.

The Z80 processor was replaced with a U 880 (which is 100% bug-compatible), and some proprietary Amstrad I/O chips replaced with clones based on the Z8536. This clone machine is around 95% compatible with the original.

The CPC 5512

The CPC 5512 was a non-functional fake made by a French magazine featuring 512K RAM, 5.25" floppy disc drive and GEM on a CPC 6128 clone.

PLUS models

In 1990 Amstrad introduced the "Plus" series which tweaked the hardware in many ways and added added a cartridge slot to all models. Most improvements were to the video display which saw an increase in palette to 4096 colours and gained a capacity for hardware sprites. Splitting the display into two separate windows and pixel scrolling both became full supported hardware features although both were possible on the non-"Plus" hardware using clever programming of the existing Motorola 6845.

An automatic DMA transfer system for feeding the sound chip was also added but the sound chip itself remained unchanged. Additionally, the BASIC command set for disc access was improved.

A cut down CPC+ with the keyboard and support for non-catridge media was released simultaneously as the GX4000 video game console.

These models did not do very well in the marketplace, failing to attract any substantial third party support. The 8-bit technology behind the CPC was starting to look a little out of date by 1990 and users resented the substantial price hike for cartridge games compared to their tape and disc counterparts.

Hardware & Software

Overview

All CPC models were based on a Zilog Z80 processor clocked at 4 MHz but usually running a little slower due to the requirements of sharing RAM with the video circuits. (Any Z 80 instruction was forcibly blown up to multiples of 4 clocks.)

The system came with 64 KB or 128 KB of RAM depending on the model. The machines also featured a standard 9pin Atari style joystick socket.

Video and graphics

Underlying the CPCs video output was the Motorola 6845 address generator. This was connected to a pixel generator that supported 4bpp, 2bpp and 1bpp output. The address generator was clocked at a constant rate so the 4bpp display generates half as many pixels as the 2bpp as a quarter as many as 1bpp.

The ROM featured three built in display resolutions but many others could be achieved by reprogramming of the 6845.

The standard video modes were:

  • Mode 0: 160x200 pixels with 16 colors (4 bits per pixel)
  • Mode 1: 320x200 pixels with 4 colors (2 bits per pixel)
  • Mode 2: 640x200 pixels with 2 colors (1 bit per pixel)

A colour palette of 27 colors was supported, derived from RGB colour space with each component assigned as either off, half on or on. The later Plus models extended this to 4096 colours and added support for hardware sprites.

This hardware compares well with the other 8-bit computers. In particular the CPC lacks the colour clash of the ZX Spectrum and clever programming of the 6845 could produce overscan, different resolutions (although with the same pixel density) and smooth pixel scrolling.

The machine lacked either an RF TV or composite video output and instead shipped with a proprietary 5-pin DIN connector intended for use solely with the supplied Amstrad monitor. An adapter for RF TV was available separately.

The 5-pin DIN connector is capable of driving a SCART television with a correctly wired lead.

Audio and sound

The CPC used the Yamaha AY-3-8912 as a sound generator, providing three channels, each configurable to generate square waves, white noise or both. A small array of hardware volume envelopes are available.

Output was provided in mono by a small (4 cm) built-in loudspeaker with volume control, driven by an unusually powerful amplifier. Stereo output was provided through a 3.5mm headphones jack, not present on some early CPC464 models. In those models, what looked like a standard 3.5" headphone jack was actually used for connecting an external tape recorder, although later models used a 5-pin DIN connector for the same purpose.

Through clever programming of the sound chip playback of Digital Sound samples at a resolution of a little better than 5-bit was possible, as heard on the title screen of the game Robocop. This trick was very processor intensive and hard to combine with any other processing.

Built-in BASIC and operating system

Like most home computers at the time, the CPC had its OS and a BASIC interpreter built in as ROM. It used Locomotive BASIC - a variant specifically written for the CPC hardware which as a result was faster, more comfortable and more powerful than the generic but common Microsoft BASIC used by the Commodore 64 and MSX amongst others. It was particularly notable for providing easy access to the machine's video and audio resources in contrast to the arcane POKE commands required on Microsoft implementations.

Other Languages

Although it was possible to obtain compilers for Locomotive BASIC, C and Pascal the majority of the CPC's software was written in native Z80a assembly language.

An interpreter for the educational language LOGO was also available.

Criticism of CPC's software (games)

The quality of CPC software has been frequently criticized as being rather low compared to the machine's potential, and often didn't measure up to the equivalent ZX Spectrum or C64 implementations.

Because the CPC shared the Z80a processor with Sinclair Research's ZX Spectrum and could be made to produce a similar display from the same code, many games manufacturers developed games for the two systems in parallel or ported older Spectrum games yielding products that did not take advantage of hardware scrolling or the availability of 4 and 16 colour modes.

For those actually targeting the CPC the hardware lacked support for sprites and scrolling required some careful memory arrangement and was for a long time thought impossible to do smoothly in the vertical direction. This made the creation of smooth-running and colourful games harder. Titles from the late 80s onwards tended to be coded more carefully than their mid 80s counterparts, making better use advantage of the machines graphics capabilities, featuring smooth scrolling and better color usage.

Although the machine received more software support than most other Z80-based systems, the overall impression left is that said support could have been a lot better in a lot of cases, and the commercial success of the system could have been completely different. Only a handful of game titles, such as Gryzor or Turrican can be said to have taken full advantage of the system's capabilities, let alone taken it "to the edge", like it was common with many C64 and ZX Spectrum titles.

Notable Games

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Cybernoid_2_cpc.PNG
Cybernoid 2
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Turrican
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Crafton&Xunk
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Switchblade


The 3" floppy drive

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Compact_Floppy.jpg
The CF has a harder casing than a 3½" floppy; the metal door is opened by a sliding plastic tab on the right side.

Amstrad's idiosyncratic choice of Hitachi's 3" floppy disc drive, when the rest of the PC industry was moving to Sony's 3.5" format, is often claimed to due to Amstrad bulk-buying a large consignment of 3" drive units in Asia. The discs were reversible; in the cheapest drive, a single-sided 40-track unit, the disk could be removed and re-inserted into the drive the other way up. As the discs were designed for this use, they sported 2 indpendent write-protect switches. The sides were termed "A" and "B" and each side held 180KB for a total of 360KB per disc.

Disks were shipped in a paper sleeve or a hard plastic case resembling a Compact Disc "jewel" case. The casing is thicker and more rigid than that of 3.5" diskettes and sliding metal cover to protect the media surface is internal to the casing and latched, unlike the simple external sliding cover of Sony's version. Because of this they were significantly more expensive than both 5.25" and 3.5" alternatives. This, combined with their low nominal capacities and their essentially proprietary nature, lead to the format being discontinued when the CPC itself was discontinued.

Apart from Amstrad's other 3" machines (the PCW and ZX Spectrum +3), the only other computer systems to use them were mostly obscure and exotic CP/M systems such as the Tatung Einstein and Osborne machines.

External floppy drives

The data formatting of 3" disks reproduced that of 5¼" disks on a smaller scale, and the Amstrad CPC machines were able to use 5¼" drives through their "external drive" port - either one specially designed for use by the CPC or an adapted IBM-PC drive.

A less popular alternative was to attach an adapted IBM-PC 90 mm (3½") drive for operation in either single-sided 180 KB or double-sided 360 KB mode.

Similarities to the BBC Micro

The CPC has been called an improved Z80 implementation of the earlier BBC Micro due to similarities in firmware and hardware. Both use the Motorola 6845 video address generator and the two have very similar sound output chips - the AY in the CPC provides three tone channels each optionally with added noise and the Texas Instruments SN76489 in the BBC offers three tone channels and one exclusively noise channel.

The BBC Micro uses an Intel 8271 floppy disc controller. The CPC uses the 8272, which is the 8271 with the addition of a double density (MFM) mode.

The "two cursor" BASIC editing system seen on the Amstrad CPC whereby holding shift and using the cursor keys moves a shadow text cursor allowing text to be copied from another area of the screen to the normal cursor is a lift from BBC BASIC, albeit substantially improved by allowing free movement of the normal cursor.

Both systems provide similar sytstems of full hardware abstraction through OS calls that save programs which don't require time critical hardware access from having to touch the underlying machine and provide a level of machine portability for those programs.

Magazines

Magazines available for the system (at various times) included Amtix, Computing With The Amstrad, Amstrad Computer User (Amstrad official publication), Amstrad Action, and CPC Attack. Notable games include Bloodwych, the Rick Dangerous, Turrican and Dizzy series, and Head over Heels.

Later Amstrad

Ultimately, the company purchased Sinclair Research, discontinuing the unsuccessful Sinclair QL 16-bit business machine and relaunching the 128KB Sinclair Spectrum in "+2" and "+3" variants with better keyboards and integral storage. The case and design of these recognisably drew from the CPC series.

Hardware Tricks on the CPC series

Simple reprogramming of the Motorola 6845 could produce extended graphics modes up to 784*384 pixels through overscan. Careful timing of palette switches could theoretically allow all 27 hardware colours to be visible in any display mode but due to CPU burden programs with unpredictable on screen motion such as games would typically only find time to change palette once or twice per frame.

The CRTC could also be tricked into providing splitting the display along any horizontal line, allowing the video address to be arbitrarily changed at that point. This meant that it was possible to hardware scroll portions of the display while leaving others static.

The machine had an internal mechanical relay for controlling the tape recorder's motor which when switched would produce an audible click. Some demos used this trick to produce "realistic" percussion sounds, but usually such an abuse of the relay resulted in early failure.

Careful programming of the AY sound chip could cause it to produce a level wave. Adjustment of the output volume would cause adjustments in the height of the wave. Using this observation it was possible to output slightly better than 5 bit digital audio, albeit at a very high CPU cost.

See also

fr:Amstrad CPC 464

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