MGT Sam Coupé

Pubblicato: giugno 24, 2011 in Sinclair
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From Wikipedia, the free encyclopedia

The SAM Coupé (Pronounced: “Sam Koo-Pay” from its original British English branding) is an 8-bit British home computer that was first released in late 1989. It is commonly considered a clone of the Sinclair ZX Spectrum computer, since it features a compatible screen mode and emulated compatibility, and it was marketed as a logical upgrade from the Spectrum. It was originally manufactured by Miles Gordon Technology (MGT), based in Swansea in the United Kingdom.

Hardware overview

The machine is based around a Z80B CPU[1] clocked at 6 MHz, and contains an ASIC that is comparable to the Spectrum’s ULA. Memory is accessible within the 64 KB range of the Z80B CPU by slicing it into 16 KB blocks and accessing IO ports to switch which blocks appeared in the 4 slots available to the CPU. The basic model has 256 KB of RAM, upgradable internally to 512 KB and externally with an additional 4 MB (added in 1 MB packs). The computer’s primary storage medium is a cassette tape, though one or two 3.5 inch floppy disk drives can be installed internally as well. Six channel, 8 octave stereo sound is provided by a Philips SAA 1099 chip. The ASIC also includes a line triggered interrupt counter, allowing video effects to be synchronised to specific display positions with little effort.

The SAM Coupé provides four graphics modes:

  • Mode 4 — 256×192, linear framebuffer, 4 bits per pixel (16 colours) = 24 KB
  • Mode 3 — 512×192, linear framebuffer, 2 bits per pixel (4 colours) = 24 KB
  • Mode 2 — 256×192, linear framebuffer, 1 bit per pixel with 32×192 separate attributes for each 8×1 block of pixels = 12 KB
  • Mode 1 — 256×192, separate attributes, non-linear framebuffer arranged to match the display of the ZX Spectrum = 6.75 KB

All modes are paletted, with a 16-entry CLUT selecting from a palette of 128 colours. Palette entries consist of 2 bits for each of the red, green and blue components as well as an extra bit which slightly increases the intensity of all three components. The machine’s non-standard SCART connector includes signals to drive a TTL-style monitor, in which case the total palette of colours is reduced to 16.

In order to match the display speed of the ZX Spectrum, the Coupé introduces extra wait states to reduce the CPU speed while in Display Mode 1.

The Motorola MC1377P RGB to PAL/NTSC encoder creates a composite video signal from the machine’s RGB- and Sync-signals (output by the ASIC) for the RF modulator.

The machine shipped with 32 KB of ROM containing code to boot the machine and a BASIC interpreter (SAM BASIC) written by Andrew Wright and heavily influenced by his earlier Beta BASIC for the ZX Spectrum. No DOS was included in the ROMs, this was instead loaded from disk using the BOOT or BOOT 1 command, or the F9 key. The majority of disks shipped with SAMDOS, the system’s first DOS, on them so that they could be directly booted. An improved replacement, MasterDOS, was also developed offering faster disk access, more files and support for the Real Time Clock for filestamps amongst many other improvements.

The BASIC was very advanced and included code for sprite drawing and basic vector shapes such as lines and circles. The screen co-ordinate system for these was variable and could be arbitrarily scaled and centred. A provision for “recording” sequences of graphics commands so that they could later be repeated without the speed penalty of a BASIC interpreter in between, very similar to the display lists of OpenGL, was provided.

Video memory problems

Internal RAM was shared between the video circuitry and the CPU, with CPU accesses incurring a speed penalty (the memory contention delay) as it waited for ASIC accesses to finish. As a result, the SAM Coupé’s CPU ran only around 14% faster than the ZX Spectrum CPU, yet was required to do much more work in SAM’s appealing high-resolution modes to produce a similar movement on the display. A Mode 3 or Mode 4 screen uses four times as much RAM as a ZX Spectrum, so four times the work had to be done in the same time.

A small compensation was the straightforward arrangement of colour pixels in this memory, instead of the ZX Spectrum’s oddly-laid-out display and attributes memory. Low-level graphics software operations could be much simpler than their Spectrum equivalents and therefore somewhat faster to execute.

The penalty of memory contention delay applied to all memory accesses to RAM, and not just to memory associated with the video circuitry (as in the case of the ZX Spectrum). Hardware sprites and scrolling would have greatly improved the performance of games, unfortunately there was insufficient wafer space on the VLSI ASIC to include such circuitry.

While the main 256×192 area of the screen was being drawn, the processor could only access memory in 1 out of every 8 t-states. During the border area this was 1 out of every 4 t-states, which had no effect on the many instructions whose timings were a multiple of 4. In modes 3 and 4 the display could be disabled completely, eliminating these memory contention delays for a full 6 MHz running speed. Code running in ROM was unaffected by the contention, though any RAM accesses they performed would still be affected.

Disk drives

The SAM used Citizen 3.5 inch slimline drives which slotted in below the keyboard to provide front-facing slots. Like IDE hard disks, these enclosures contained not just the drives but also the drive controllers, a WD1772-02, with the effect that the SAM could use both drives simultaneously.

Due to a flaw in the Coupé’s design, resetting the machine while a disk was left in a drive would be liable to cause data corruption on that disk. With the appropriate technical expertise, this fault was easily corrected.

The double density disks used a format of 2 sides, 80 tracks per side and 10 sectors per track, with 512 bytes per sector. This gave a total capacity of 800 KB, though the standard directory occupied 20 KB leaving 780 KB free for user files. Files were stored in the same structure as MGT’s original +D interface, but with additional codes used for SAM Coupé file types. The firmware of the disk controllers was compatible with that for IBM PC, and programs were available to read FAT formatted disks.

Expansion ports

A large array of expansion ports were provided, including:

  • Two internal drive bays.
  • Slightly non-standard SCART connector offering composite video and digital and linear RGB.
  • 64-pin Euroconnector for general purpose hardware expansions.
  • Mouse socket (proprietary format, although a converter for Atari ST style mice was later available).
  • Lightpen / Lightgun via 5-pin DIN.
  • MIDI IN/OUT ports (and THROUGH, via a software switch).
  • Network using the MIDI port (up to 16 machines could be interconnected).
  • Atari-style 9-pin joystick port (dual capability with a splitter cable).
  • 3.5 mm mono Cassette jack.
  • Stereo sound output through 5-pin DIN.

Uniquely the SAM’s RF modulator was built into the power supply unit and connected via a joint power/TV socket. This made signal interference from the ACDC converter common and it was a popular but entirely unofficial modification to remove the modulator and keep it as a separate unit.

Due to a flaw in the design, when two joysticks were used at the same time (through the approved splitter) they would interfere with each other.

Up to four devices could be connected to the Coupé’s Euroconnector port, through the use of the SAMBUS, which also provided a built-in clock. When using more power-hungry peripherals, the SAMBUS required an additional power supply.

Kaleidoscope

The Kaleidoscope, announced by SAMCo shortly before bankruptcy, extended the machine’s total colour palette to 32768 colours in such a way as to allow forwards and backwards compatibility by applications. Although complete, very few were produced and the design ceased with SAMCo.

ZX Spectrum compatibility

Emulation of the ZX Spectrum was limited to the 48K and was achieved by loading a copy of the ZX Spectrum ROM and switching to display mode 1, which mimicked the ZX Spectrum display mode and approximated that machines processor speed. The ROM was not supplied with the machine and had to be obtained from a real ZX Spectrum.

The 128K model’s memory map was incompatible with the Coupé’s memory model and the machine featured an entirely different sound generator. It was possible to convert games by hacking the 128K code.

Because the Coupé didn’t run at exactly the same speed as the Spectrum even in emulation mode, many anti-piracy tape loaders would not work on the Coupé hardware. This led to the development by MGT of a special hardware interface called the Messenger which could capture the state of a connected ZX Spectrum to SAM Coupé disk for playback later without the Spectrum connected. The Messenger plugged into the Coupé’s network port, and the Spectrum’s expansion slot. Due to faulty break (NMI) buttons (needed to activate the Messenger software), a break-button card was also provided, which plugged into the Coupé’s expansion slot.

Commercial fortunes

Three different companies have owned the rights to the SAM Coupé. It is believed that about 12,000 SAM Coupé and SAM Élite machines were sold in total.[2]

Miles Gordon Technology, plc.

MGT, Miles Gordon Technology, plc., which originally produced add-ons for the ZX Spectrum, launched the SAM Coupé (very) late in 1989, missing the Christmas sales. They ended up with a vast number of machines in stock. The 16-bit and PC markets were on the rise and it helped little that MGT in the beginning of 1990 had to ship a new ROM to about 8,000 existing customers to fix bugs, notably a DOS booting bug. MGT went into receivership in June 1990.

SAM Computers Ltd.

Immediately after the collapse of MGT, the founders of the company, Alan Miles and Bruce Gordon, bought back the company’s assets and formed SAM Computers Ltd. The price of the SAM with floppy disk drive was brought down to under £200 and new games and hardware were released. SAMCo survived until 15 July 1992.

West Coast Computers

Stock from SAM Computers Ltd. were bought by West Coast Computers in November 1992. They revamped the SAM Coupé into SAM Élite. The only changes made were that 512 KB became standard and an external printer connector was added. The slim-line floppy drives from Citizen, which had withdrawn them from the European market in 1990, were replaced with standard 3.5 inch drives. Little is known about the company. For a long period the only point of contact was Format Publications, run by Bob Brenchley, which faded out of existence sometime around 1998.

SAM the robot

Devised by Mel Croucher and put in pen by Robin Evans as a mascot for the machine, SAM the robot appeared in the user manual and on most of the advertising literature for the machine, and later made an appearance as the main character in the game SAM Strikes Out!.

Notable software

The SAM Coupé was particularly notable for the wide array of disk based magazines that originated for it, include FRED and the official SAMCo Newsdisk. It also became notorious for the overwhelming number of puzzle games for the system, something that Spectrum magazine Your Sinclair jokingly referred to on numerous occasions.

Several famous computer games were ported to the SAM, notably Manic Miner, Prince of Persia, and Lemmings. An unofficial but arcade perfect port of Defender surfaced late in the machine’s lifespan.

Flash!

Flash!, an art package, was the only full application bundled with every SAM Coupé and as a result is probably the program best known to SAM owners. Written by Bo Jangeborg, author of the earlier ZX Spectrum program The Artist and The Artist II, it offered pixel editing in all four graphics modes, conversion of graphics from one mode to another and some basic animation functions.

Only full screen images were supported and the program’s main flaw was an inability to view the entirety of an image while working on it. A copy adapted for use with a mouse was bundled with the official mouse addon.

Software houses

Before the machine was released, US Gold claimed that “if, as with Strider, we’ve already produced a game across all common formats, all we have to do is simply take the code from the Speccy version and the graphics from the ST and sort of mix them together. This should take one bloke around two weeks at most”.

Despite this, supporting the machine proved difficult and only a small number of software houses stepped forward, most SAM specific.

Enigma Variations

An early supporter of the SAM, Enigma published SAM versions of Defenders of the Earth, Escape From the Planet of the Robot Monsters, Five on a Treasure Island (based on Enid Blyton’s Famous Five), Klax, Pipe Mania and SAM originals SAM Strikes Out (a Jet Set Willy influenced platformer), Futureball (a Speedball influenced futuristic sporting title) and Sphera.

Revelation

The software arm of SAMCo, founded in 1992 due to the lack of support from mainstream publishing houses, was notable for publishing most of the SAM’s best titles. SAM original titles included Astroball, Batz ‘n’ Balls, Legend of Eshan, Sophistry and Wop Gamma. Revelation also published Hexagonia, which is similar to Atomix.

SAM ports included Elite (nothing more than the ZX Spectrum 48K version repackaged onto floppy disk), Prince of Persia, Manic Miner, Lemmings and Splat!.

A later incarnation of Revelation was set up in conjunction with West Coast Computers, with titles distributed by Format Publications.

FRED Publishing

Spun off from the disk based magazine, FRED Publishing was relatively late to the scene, but supported the machine long after any of the other publishing houses. The jewel in its crown was the SAM conversion of Lemmings (and Oh No! More Lemmings), but they also published a number of SAM original titles such as Boing, The Bulgulators, Dyzonium, Football League Manager, Impatience/Triltex, Momentum, Parallax, Waterworks and Witching Hour.

Phoenix Software

This label released titles such as Manic Miner and Dyadic. They also distributed titles for other authors, such as MasterBasic and MasterDos. The same team was also involved with SAM Prime magazine.

Quazar

A relatively late comer to the SAM scene, founded in 1995. Launching with a new soundcard for the SAM and continued producing a disk magazine to support it but later spanned over into games including Stratosphere and the Money Bags trilogy. Still actively producing software, hardware and a regular magazine for the SAM Coupe.

 

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