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| Manufacturer | Miles Gordon Technology |
|---|---|
| Type | Home computer |
| Generation | 8-bit |
| Released | 1989; 37 years ago (1989) |
| Discontinued | 1992 |
| Units sold | 12,000 |
| Operating system | SAM BASIC,CP/M via software |
| CPU | Zilog Z80B @ 6 MHz |
| Memory | 256 KB/512 KB (4.5MB max.) |
| Removable storage | 3.5-inch floppy disk,Cassette tape |
| Display | UHF TV, composite or RGB monitor; 512 × 192 with 4 colors, 256 × 192 with 16 colors |
| Graphics | Motorola MC 1377P, VGT-200 gate array custom ASIC graphicprocessor |
| Sound | Philips SAA1099 (6 channel, 8 octave, stereo, envelope and amplitude control); Beeper |
| Input | Mouse, Light pen, Light gun, joystick |
| Connectivity | SCART, 64-pin Euroconnector, Mouse port, Light pen / Light gun port, 5-pin DIN stereo audio out, MIDI ports, 9-pin joystick port, 3.5 mm mono audio in/out |
| Backward compatibility | ZX Spectrum |
TheSAM Coupé (pronounced /sæm ku:peɪ/ from its originalBritish English branding) is an8-bitBritishhome computer manufactured byMiles Gordon Technology (MGT), based inSwansea in theUnited Kingdom and released in December 1989.
It was based on and designed to have a compatibility mode with theZX Spectrum 48K with influences from theLoki project[1] and marketed as a logicalupgrade from the Spectrum with increased memory, graphical and sound capabilities, native peripheral support (floppy disk,MIDI,joystick,light pen/light gun and a proprietarymouse).
The inclusion of support for higher graphical modes allowed for80-column text presentation, providing a platform to supportproductivity andCP/M applications via additional software.
Being based on8-bit technology at a time when16-bit home computers were more prevalent, coupled with a lack of commercial software titles, led to it being a commercial failure.[2][3]
WhenMGT went into receivership in June 1990[4] two further attempts were made to restart the computer and brand, firstly underSAM Computers Limited[5] and then in November 1992 underWest Coast Computers, a company spun fromFormat Publications which lasted until liquidation in 2005.[6]
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The capitalised SAM is an acronym for 'Some Amazing Micro' according toAlan Miles in an interview with ZAT magazine.[7]
The ‘Coupé’ part has two sources: one being anice cream sundae called the “Ice Cream Coupé” and the other because the machine resembles afastback car in profile with the feet as the wheels.[8][9]
The SAM Coupé's hardware was designed by Bruce Gordon ofMiles Gordon Technology. The computer included custom silicon to handle display, memory and IO functionality. This was originally prototyped usingwire-wrapped7400-series logic chips, before being produced as aVLSI VGT-200 gate arrayASIC.[10][11]
The machine is based around aZ80BCPU clocked at 6 MHz and a 10,000-gateASIC. The ASIC performs a similar role in the computer to theULA in the ZX Spectrum. The Z80B CPU accesses selected parts of the large memory space in its 64 KB address space by slicing it into 16 KB banks and using I/O registers to select the memory pages mapped into each 16 KB bank.
The basic SAM Coupé model has 256 KiB ofRAM, internally upgradable to 512 KiB via a connector on the main board accessible via a trapdoor underneath, and externally up to an additional 4 MiB, added in 1 MiB packs via theEuroconnector[12] on the back of the system.
The computer has a direct connection for a cassette recorder for data storage but two 3.5 inchfloppy disk drives can be installed within the case as well or externally using an interface.
The SAM Coupé was designed primarily for theUK market, and is designed around thePAL television standard, which refreshes at 50 frames per second. Unlike a standard PAL signal which is interleaved, the SAM is designed to emit two identically positioned fields at approximately 50.08 FPS, giving something closer to a 312 linesprogressive signal than the625-linesinterlaced broadcast television signals common in the UK at the time.
The display is surrounded by a large border area to provide a title-safe display zone for the CRT televisions of that era. The colour of this region can be changed in software by using the BORDER port to select a colour from the palette.
The SAM Coupé has four display modes:[13]
The 'attribute' modes borrow their design from theZX Spectrum, where a bitmap is used to select between a paper and ink colour from two groups of eight colours. The group to use is selected by a 'brightness' flag. The colour block can also be set to 'flash' - that is, alternate between the two colours used for paper and ink.
| Bit 7 | Bit 6 | Bit 5 | Bit 4 | Bit 3 | Bit 2 | Bit 1 | Bit 0 |
|---|---|---|---|---|---|---|---|
| Flash | Bright | Paper 2 | Paper 1 | Paper 0 | Ink 2 | Ink 1 | Ink 0 |
All modes use palette-basedcolour look-up tables, selecting from apalette of 128 colours. The palette values consist of 2 bits for each of the red, green and blue components as well as an extra bit which increases the intensity of all three components by a half-step (a 'brightness' bit).
| Bit 7 | Bit 6 | Bit 5 | Bit 4 | Bit 3 | Bit 2 | Bit 1 | Bit 0 |
|---|---|---|---|---|---|---|---|
| - | Green 1 | Red 1 | Blue 1 | Half-Bright | Green 0 | Red 0 | Blue 0 |
TheASIC can be configured to generate interrupts when a line on the display is starting to be emitted, allowing video effects to be synchronised with specific display lines with little effort. By default, it will generate an interrupt for every frame. Typically this interrupt is used to double buffer the frame, read the keyboard/mouse state, and output music.
The interrupt state can also be polled directly from the ASIC's status register.
TheMotorola MC1377PRGB toPAL/NTSCvideo encoder creates acomposite video signal from the machine's RGB- andSync-signals[14] (output by the ASIC) for theRF modulator.[15][11]
The non-standardSCART display connector includes both composite and RGB output, as well as signals to drive a 16-colourTTL monitor.
Access to internal RAM was shared between the display and the CPU, with CPU accesses incurring a speed penalty (memory contention) as it was forced to wait for isochronousASIC memory-accesses to complete. As a result, the SAM Coupé's CPU effectively ran only around 14% faster than the ZX Spectrum CPU, yet was required to do much more work in SAM's 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 Mode 1 display, so four times the work had to be done in the same time when updating it.
A small compensation was the straightforward arrangement of colour pixels in this memory, instead of the ZX Spectrum's more limited 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). Hardwaresprites andscrolling would have greatly improved the performance of games, unfortunately there was insufficientwafer space on theVLSI 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 or external RAM was unaffected by contention, though any RAM accesses they performed to shared internal RAM would still be affected.
The character set includesblock graphics andinternational characters. Font size can be altered to make text display in 32, 64, or 85 columns, and double height characters are also possible.[13][16] Users can freely define their own characters by creatingUser Defined Graphics (UDGs). A large number of character codes (144 to 255) are available for this purpose. Block graphics (code 128 to 143) can be turned off in order free more characters forUDGs.[16]
When in BASIC, the cursor is composed of the Blocks off characters (128/129) for lower and upper case respectively regardless of current Blocks setting, so overwriting these positions will also change the cursor. If this is undesirable, then systems variable (SVAR) 1 can be set to a 2-character value for the new lower and upper case cursor characters to be used.
The Utility Tapes and SAMDOS disk operating system media included an international character font loader to allow additional characters to be loaded into the block graphics region starting at 128 (0x80). This meant the cursor characters were overwritten so the accompanying BASIC loader set the cursor to * and + for lower and upper case respectively.[17]
The following table show the predefined character set arrangement:[13][16]
| 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | A | B | C | D | E | F | |||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Control Codes | 0 | |||||||||||||||||
| 1 | ||||||||||||||||||
| Predefined Character Set | 2 | ! | " | # | $ | % | & | ' | ( | ) | * | + | , | - | . | / | ||
| 3 | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | : | ; | < | = | > | ? | ||
| 4 | @ | A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | ||
| 5 | P | Q | R | S | T | U | V | W | X | Y | Z | [ | \ | ] | ↑ | _ | ||
| 6 | £ | a | b | c | d | e | f | g | h | i | j | k | l | m | n | o | ||
| 7 | p | q | r | s | t | u | v | w | x | y | z | { | | | } | ~ | © | ||
| Block graphics / Foreign characters | Blocks on | 8 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
| Blocks off |  |  | é | |||||||||||||||
| Blocks off, International characters | É | æ | Æ | ô | ö | ò | û | ù | ÿ | Ö | Ü | ¢ | £ | ¥ | ₧ | ƒ | ||
| User Defined Graphics | Blocks on | 9 | ||||||||||||||||
| Block graphics / Foreign characters | Blocks off, International characters | á | í | ó | ú | ñ | Ñ | ª | º | ¿ | ||||||||
| User Defined Graphics | A | |||||||||||||||||
| B | ||||||||||||||||||
| C | ||||||||||||||||||
| D | ||||||||||||||||||
| E | ||||||||||||||||||
| F | ||||||||||||||||||
Six channels of 8-octavestereo sound are provided by aPhilips SAA1099 sound and noise generator chip. The machine allows for the playback of sound samples under software control as well as supporting four-channelAmigaTracker modules at 3-bit sample resolution of 10.4 kHz[18] and 4-bit at 15.625 kHz.[19]
To provide backwards compatibility with theZX Spectrum, the SAM also provides a single-bit 'beeper' channel which can be used to emit simple tones by toggling the bit on and off as per the original Spectrum.
The machine shipped with 32 KB ofROM containing code to boot the machine and aBASIC interpreter (SAM BASIC) written by Andrew Wright and heavily influenced by his earlierBeta BASIC for the ZX Spectrum. The ROMs contained only the bootstrap code and theDOS was instead loaded from disk using theBOOT command, or theF9 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 thereal-time clock accessory to provide file timestamps amongst many other improvements.
The BASIC was very advanced and included code forsprite drawing and basicvector 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 was provided.
The machine is capable of runningCP/M 2.2 using the Pro-Dos software with support for both 720 Kilobyte format disks andIDE drives[20]
The SAM originally usedCitizen 3.5 inch slimline drives which slotted in below the keyboard to provide front-facing slots. LikeIDE hard disks, these enclosures contained not just the drives but also the drive controllers, aWD1772-02, 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 causedata corruption on that disk, as while the RESET line is held logic low the ASIC (that generates the 8 Mhz clock) is halted thus no clock signal is sent to the drive's controllers.[22]
Thedouble 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 disk encoding (NRZ), encoding strategy (linear angular velocity), and track and sector header formats were compatible with those used on theIBM PC andAtari ST, and programs were available to readFAT formatted disks.[23][24]
A large array of expansion ports were provided, including:
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.[25]
The SAM's Power Supply was a modifiedAmstrad CPCMP1/MP2 modulator unit, with the RF modulator built in and connected via a joint power/TV socket to the computer. This made signalinterference from the AC/DC converter common and it was a popular but entirely unofficial modification to remove the modulator and keep it as a separate unit.[26]
Emulation of the ZX Spectrum was limited to the 48K and was achieved by either using a supplied utility and a skeleton ROM image (containing no original code) or by loading a complete copy of the ZX SpectrumROM (obtained from a ZX Spectrum). In order to match the processing speed of the ZX Spectrum (3.5 MHz), the SAM Coupé introduces extra wait states in display Mode 1 (the ZX Spectrum-compatible graphics mode) to slow down the CPU to roughly match the rate of that system.
The128K model's memory map wasincompatible with the Coupé's memory model and the machine featured an entirely different sound generator. It was possible to convert some games and demos by directlyhacking the 128K code or by using a softwarepatcher with the 1megabyte RAM expansion to provide the address space.[27]
Because the Coupé did not 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 hardwareinterface called theMessenger 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 unsuitable onboard break (NMI) buttons (needed to activate the Messenger software), ade-bounced break-button card was also provided, which plugged into the Coupé's expansion slot.
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