Arnold V specs

ARNOLD V TECHNICAL SPECIFICATION

full technical details for the Amstrad Plus range

PRODUCT RANGE OVERVIEW

This project provides a more sophisticated and stylish replacement for the earlier CPC464 and CPC6128 computers. This has been achieved by:

Redesigning the ASIC and main PCB to incorporate a number of new features
Restyling the casework to provide a more modern appearance.

Common Features

The casework consists of a new two piece set of plastic mouldings. This contains a horizontally mounted, double-sided PCB assembly on which are mounted most of the electronics for the computer. A small, vertically mounted , daughter PCB provides the connector for a ROM cartridge. Any size ROM cartridge from 16k x 8 up to 512k x 8 can be installed. The firmware, fitted to the main PCB on earlier CPC computers, is supplied instead in a ROM cartridge.

All expansion and peripheral device connectors are mounted on the main PCB. In addition to the connectors used on the existing CPC range, there are:

Separate connectors for two joysticks, replacing the old daisy-chain arrangement. However, the daisy chain system can still be used on the Joystick 1 connector if required.
An additional 15-way female D-type connector will provide four analogue input channels and access to the four existing "fire" buttons. This is pin compatible with the games control port on the PC200 (PC-8) computer.
All PCB edge connectors have been replaced by types that are easier to screen against spurious RF emission. The printer connector is a 25-way female D type, as used on the PC1640 etc, and the expansion connector is a 50-way Delta (Centronics style) type, as used for the earlier CPC range in Germany.

The 6128's TAPE socket has been replaced by a 6 pin RJ-11 type for the light gun.

The computer provides stereo sound via additional pins on the monitor connector, as well as from the stereo sound socket.

All existing CPC electrical features are provided, plus some new features. There is complete backward compatibility except that:

The border colour is undefined at power-on reset
The new 6128 version has no tape socket

The following new features become available once a software "lock" has been opened, thus preventing existing CPC software from accidentally invoking them:

16 Sprites, each consisting of 16x16 high resolution pixels, in fifteen colours separate from the main screen colours. Each sprite can be magnified in X or Y, moved around the screen , and turned on or off independent of the main screen. Sprite pixels can be transparent, and sprites have a fixed order of priority (i.e. "depth"), so that they can pass in front of each other, in front of the main screen, and behind the border.
The colour palette has been extended to allow simultaneous display of up to 32 colours (16 main + 15 sprite + border) from a palette of 4096, rather than the previous 17 from 27.
Additional screen controls have been added, to allow split screen operation and smooth scrolling to be used.
An automated sound generation process allows generation of more complex sound effects with greatly reduced software overhead.
Some other internal features to ease implementation of better games software, described in the technical specifications section.

The functions of display monitor and power supply are provided by either:

A restyled range of monitors, consisting of a white tube monochrome monitor MM12 and an improved colour monitor CM14.
An MP2-style modulator/power supply unit.
A Peritel adaptor/power supply unit.

The old CPC6128 keyboard is used, except that the colour scheme has been changed and the connecting cable exits in a different location.

Amstrad 464 Plus

This variant has an integral cassette tape drive, and 64k bytes of dynamic RAM. It is supplied with a ROM cartridge containing the system firmware plus the BASIC language, disk firmware and a game, although it is not possible to select the disk firmware.

Amstrad 6128 Plus

This variant will have an integral 3" floppy disk drive (5V) plus a 36-way Delta (Centronics style) expansion socket allowing a second 3" drive to be added. The 6128 Plus is to be supplied with a ROM cartridge containing the system firmware plus the BASIC language, disk firmware and a game. 128k bytes of dynamic RAM are fitted to the main PCB.

Further Variants

Unlike the existing CPC range, the size of dynamic RAM and whether or not a disk drive is installed are separately configurable options. It is therefore possible to produce a "4128" (128k diskless) or "664" (64k with disk) variant. Also, it is possible to increase the number of analogue input channels to eight.

TECHNICAL SPECIFICATIONS

The technical specification is essentially similar to the earlier CPC 464/6128 range, with some enhancements. This specification should therefore be read in conjunction the "Amstrad CPC 6128 Software Interface Spec" Issue 2, 17th February 1985. New features have been added by changes to the ASIC and main PCB circuitry.

The overriding concern in the specification of this new product range has been the need for total backward compatibility with the existing CPC range. Many of the new features within the ASIC employ new registers, which can be mapped to replace the page of RAM from 4000 to 7FFFh in the CPU memory map, by setting a bit pattern in an I/O port. Before this port is allowed to "exist", a deliberately obscure I/O sequence is needed. This mechanism protects existing CPC range software from accidents such as killing its own RAM page.

The following new features are provided by change to the ASIC and the main PCB electronics:

Hardware Sprites

Sixteen hardware sprites are to be provided by the ASIC.

Each consists of an array of 16x16 pixels of four bits per pixel. A sprite pixel is "transparent" when it has a value of zero, thus allowing 15 sprite colours. The sprite pixel data exists in memory mapped registers with the ASIC, from address 4000h. The lower four bits of each byte contain the data for a single pixel. The first 16 bytes contain the data for the upper scan line, starting at the top left hand corner of the sprite. 15 more similar scan lines of 16 pixels each follow, thus each 256 (0100h) byte block of register space contains one sprite. When the data for a sprite is read or written, that sprite is removed from the display for the duration of the access. Thus sprite data should only be accessed during retraced time, or while the raster is scanning somewhere else, otherwise there is a risk of disruption of the display.

The position on screen of the upper left corner of each sprite, and the X and Y magnification, are defined by five registers for each sprite:

	A2 A1 A0

	0 0 0	X position LSB

	0 0 1	X position MSB

	0 1 0	Y position (scan line) LSB

	0 1 1	Y position MSB

	1 0 0	bits 3,2 = X magnification, bits 1,0 = Y magnification

The position registers are read/write, and accept numbers in two's complement form. They should only be changed during retrace or when a sprite is off. Data written to these registers should be between +767 and -256 for X, and between +255 and -256 for Y, otherwise the sprites will appear in strange positions. With standard 6845 timing (64us scan lines, 200 visible lines), on screen positions at maximum sprite magnification are -64 to +639 in x and -63 to +199 in y. A sprite will not be displayed if either the vertical or the horizontal positions outside the onscreen range. The magnification registers are cleared to zero at reset, and are write only. They are coded as:

0 0	Sprite not displayed
0 1	Magnification x1
1 0	Magnification x2
1 1	Magnification x4

The sprite control registers exist on 8-byte boundaries from addresses 6000 to 607Fh for sprites 0 to 15 respectively.

All sprite characteristics are independent of the main screen mode, the unmagnified pixel size being as for screen mode 2 (640x200). Sprite colours are defined by 15 entries in the colour palette (see section 2.2 below). Thus sprites can be in different colours and resolutions from the rest of the screen. Sprites may overlay with each other or the border, and are prioritized so that the border has the highest priority, followed by sprites 0 to 15 in sequence, then the main screen data. Thus sprites always appear "in front of" the main screen and behind "the border".

Colour palette

The earlier colour palette within the ASIC, which selects 17 of 27 possible colours, has been replaced by a new palette which selects 32 of 4096 colours. This can be accessed through two ports. The primary port provides full access via 32 registers of 12 bits, i.e. 4 bits each for red, green and blue.

For compatibility with earlier models a secondary port provides access to the first 17 registers only (i.e. main screen colours and border), via the existing 5 bit interface. A block of logic maps the 5 bit colour written to the palette at the address selected by the "palette pointer register".

The primary palette port is between addresses 6400 and 643Fh, each pair of bytes representing one entry in the palette. The most significant byte contains the GREEN information in the lower nibble (D3-D0), and the other byte contains RED (D7-D4) and BLUE (D3-D0).

This ordering of colours has been selected to give the most consistent grey scale possible on a monochrome display (green is brighter than red, which is brighter than blue). However, because of the need to retain compatibility with the existing 27 level grey scale, the colours are summed with a 9:3:1 weighting rather than the 256:16:1 weighting which would be required to make the 12 bit word fully monotonic.

The primary palette registers appear in RAM low byte first, so that they can be loaded via a single 16-bit LD instruction, e.g. LD (6400h),0F00h would set the main colour 0 to bright green. The palette is dual ported so that there are no restrictions on when it can be accessed.

The primary port palette registers are:

	6400-641Fh	main screen colours 0 to 15

	6420-6421h	border colour

	6422-643Fh	sprite colours 1 to 15

The secondary port registers are:

	00-0F	main screen colours 0 to 15

	10-1F		border colour

Split Screen facility

Three new memory mapped registers have been added within the ASIC, to provided a horizontally split screen facility. One at address 6801h defines the scan line after which the screen split occurs. A value of zero (as at power on reset) will turn this feature off.

The other register pair at 6802h and 6803h define the start address in memory (similar to R12 and R13 respectively in the 6845, and therefore high byte first) which represents the location in memory from which to start displaying data for the lower screen. This allows the lower part of the picture to come from a separate memory area and be separately scrolled. However, note that soft scrolling (Section 2.5 below) acts on the whole screen.

Note that care should be taken with programming this facility such that the screen split does not alter the function of address bits A1-A8 and the dynamic memory refresh is not upset. This can be accomplished by setting the start of the second screen to lie on 16k boundary. The value in register pair 6802h/6803h is the first displayed line, and not the start address of the 16k block. Also, during vertical retrace, the value in register 6801h should not be set to 257 less the total number of scan lines on the screen. With a normal screen of 312 scan lines, the value 312 - 257 = 55, or 37h should not be programmed unless (1) the vertical total adjust register is set to 1 while 6801h contains 37h, or (2) the raster interrupt (see 2.4 below) should be used such that 6801h contains 0 during vertical retrace.

Programmable raster interrupt

A new 8 bit memory mapped register (PRI) has been added within the ASIC at address 6800h, which is cleared at power up. If zero, the normal raster interrupt mechanism functions as before. Otherwise, an interrupt occurs instead at the end of the scan line specified.. The PRI can be reprogrammed as required to produce multiple interrupts per frame. See section 2.7 below for general information on interrupts.

Soft scroll facility

A memory mapped 8 bit soft scroll control register (SSCR) has been added within the ASIC at 6804h, to allow scrolling of the screen by pixels rather than just by characters at present. It is cleared at reset. This soft scrolling mechanism affects the whole of the main screen , regardless of the split screen facility, but it does not affect sprites.

The lower four bits (D3-D0) of the SSCR define a horizontal delay of between 0 and 15 bits i.e. high resolution (mode 2) pixels. This shifts the screen image to the right by the value programmed , "losing" pixels behind the right border and instead displaying random data on the left. It is left to the programmer to ensure that the delay value is always a multiple of the number of bits per pixel.

The next three bits (D6-D4) will be added to the least significant three bits of the scan line address, thus determining which of the eight 2k blocks contains the data for the first scan line on the screen. the effect of this is to shift the display up by the number of scan lines programmed, "losing" what would otherwise be the first lines to be displayed, and instead appending extra lines to the bottom of the screen.

The most significant bit (D7), when set, causes the border to extend over the first two bytes (16 high resolution pixels) of each scan line, masking out the bad data caused by the horizontal soft scroll. Software which intends to use horizontal soft scroll should have this bit always set, so that the screen width does not keep changing.

Setting the SSCR to zero, as at reset, (i.e. no offsets, no border), will of course effectively disable soft scroll.

Automatic feeding of sound generator

An automatic process has been added to feed data to the sound generator from three instruction streams in main RAM without CPU intervention. Three separate channels each fetch one 16-bit instruction during horizontal retrace time. These instructions must be in usual Z-80 format, i.e. least significant bit first and must be aligned to word boundaries (i.e. address of first byte must be even). Once the three instructions have been captured , they are then executed sequentially. The maximum achievable update rate to the PSG is thus equal to the horizontal scan rate of 15.625 kHz per channel.

The available commands are :

0RDDh LOAD R,D Load 8 bit data D to PSG register R (0R015)
1NNNh PAUSE N Pause for N prescaled ticks (0<No4095)
2NNNh REPEAT N Set loop counter to N for this stream (0<No4095)and mark next instruction as loop start.
3xxxh (reserved) Do not use
4000h NOP No operation (64us idle)
4001h LOOP If loop counter non zero, loop back to the first instruction after REPEAT instruction and decrement loop counter.
4010h INT Interrupt the CPU (see section 2.7 below)
4020h STOP Stop processing the sound list.

Note that :

REPEAT Loops cannot be nested . Only one is allowed to be active per instruction stream at any time
REPEAT 0 and PAUSE 0 instructions have no effect, i.e. they are equivalent to NOP.
Control group (4xxxh) instructions can be logically ORed to produce more complex instructions, e.g. INT|STOP = h = Interrupt and stop.
The STOP instruction will leave the source address register pointing to the next instruction, so that the instruction stream can be continued after CPU intervention.
The argument field (N) of the REPEAT instruction is actually the number of times the loop is taken. The block of code between REPEAT and LOOP instructions is therefore executed N+1 times.

A DMA control and status register (DCSR) controls which channels are currently enabled, and also tells the CPU which channel is interrupting.

The channel enable bits in this register enable each "DMA" channel separately, and can be set by the CPU, and cleared by either the CPU, a STOP instruction, or power on reset. The interrupt bits are set when a channel is requesting an interrupt, and cleared when the CPU writes a "1" to the appropriate bit.

The control and status register bits are:

	D7	R	Raster interrupt (see 2.7 below)
	D6	R/W	Channel 0 interrupt
	D5	R/W	Channel 1 interrupt
	D4	R/W	Channel 2 interrupt
	D3		Unused (write 0)
	D2	R/W	Channel 2 enable
	D1	R/W	Channel 1 enable
	D0	R/W	Channel 0 enable

Each channel has a 16 bit source address register (SAR) and an 8 bit pause prescaler register (PPR). These are memory mapped, from address 6C00h, as follows:

	6C00h		Channel 0 address, LSB
	6C01h		Channel 0 address, MSB
	6C02h		Channel 0 prescaler
	6C03h		unused
	6C04-6C07h	Channel 1, as above
	6C08-6C0Bh	Channel 2, as above
	6C0F		Control and Status register

The SAR must be loaded by the CPU with a physical RAM address between 0000h and FFFEh. This means that the most significant two bits select which pages 0 to 3 of the DRAM is used, and the remaining bits are the address relative to the page start. The DMA process is not affected by the RAM or ROM mapping registers, and will always fetch data from RAM and not ROM. Note that the least significant bit of the address is ignored, and the instructions are always fetched from word boundaries.

The pause prescaler counts N+1 scan lines (where N is the value written by the CPU), giving a minimum tick of 64us, and a maximum of 16.384ms. When set nonzero by a pause instruction, the pause counter for a particular channel is decremented every tick until it reaches zero. Therefore, if the PPR is set to a value N and a PAUSE M instruction is executed, the total delay time between the instruction before the PAUSE and that following the PAUSE will be M * (N+1) * 64us. Pauses of between 64us and 67s may thus be generated.

The ASIC arbitrates accesses to the parallel interface device between the "DMA" channels and the CPU, allowing only one to access it at a time. CPU accesses to the 8255 could be held off by means of wait states for up to 8 microseconds if the "DMA" channel is currently executing a LOAD instruction. After a LOAD is executed, the ASIC must put the PSG address register back as it was before. To achieve this the 8255 parallel peripheral interface and the 74LS145 decoder have been integrated into the ASIC.

The exact timing is based on 1us cycles as follows. After the leading edge from HSYN from the 6845 there is one dead cycle followed by an instruction fetch cycle for each channel which is active (i.e. enabled and not paused). The execute cycles then follow for each active channel. All instructions execute in one cycle, except that LOAD requires at least 8 cycles. An extra cycle is added to a LOAD if the CPU is accessing the 8255, or two extra cycles if the CPU access was itself a PSG register write.

Interrupt service

The ASIC can produce interrupts from four sources: the raster interrupt and the three sound generator "DMA" channels.

Bit D7 is set if the last interrupt acknowledge was for a raster interrupt. Bits D6-D4 of the DCSR are set if interrupts from sound channels 0-2 respectively are active. For compatibility with earlier models, the raster interrupt is reset either by a CPU interrupt acknowledge cycle, or by writing a 1 to bit D4 of the mode and ROM enable register. The sound channel interrupts are cleared by writing a 1 to the relevant bit in the DCSR.

Thus interrupt service software in an environment where DMA interrupts are used must inspect these bits, giving highest priority to the raster interrupt, because this interrupt is always cleared automatically. Failure to observe this requirement may result in raster interrupts being missed. DMA interrupts must be acknowledged by writing a "1" to the relevant DCSR bit.

Enhanced ROM cartridge support

Previously, 32k of firmware ROM existed in two 16k blocks. The low block was at addresses 0000 to 3FFFh, and the high block at C000 to FFFFh. Expansion ROMs were mapped into C000 to FFFFh by writing a code to I/O address DFxxh. The disk ROM was code 0 or 7, depending on the state of an expansion signal.

The new Arnold V range has no on board ROM, but instead has a cartridge slot which can support ROM cartridges of up to 4Mbits (512k bytes, or 32 pages in 16k bytes). This means that cartridge games cannot be copied, because there is no firmware available when the game is installed. However, any software house producing a game where the intermediate state of play or high score table can be saved must produce their own driver software.

The upper 5 ROM cartridge address lines are controlled by the ASIC via the existing ROM mapping port (at DFxxh), and hence define which of the 32 pages are mapped to the upper ROM block (C000 to FFFFh). The machine is supplied with a ROM cartridge containing the firmware and BASIC, and, where applicable, the disk ROM.

For values less than 128 written to the mapping port, the "BASIC" page of the cartridge is always selected at the high ROM block address, unless the value last written to the mapping port matches the current disk ROM code (i.e. either 0 or 7), in which case the "Disk" page is selected. For values greater than 127, the lower 5 bits set the cartridge ROM page number directly, so that the cartridge may be addressed at pages 128-159 (80h-9Fh).

The earlier expansion ROM mapping scheme using port DFxxh and ROMDIS on the expansion bus, still functions. The only change is that ROMDIS can now disable the disk ROM, and selecting the disk ROM does not cause ROMDIS to be activated. An expansion card ROM mapped at any page takes priority over the same page numbers in the cartridge.

In addition, new bits are defined in the mode and ROM enable (MRER) register at I/O address 7Fxxh, Previously D7 = 1 and D6 = 0 to select this register, and D5 should be 0. This has been modified such that, if this register is written with D5 = 1, the bottom five bits are redefined. This new register is known as the secondary ROM mapping register (RMR2). D4 and D3 control the address of the low bank, and also whether the memory mappped registers are enabled at 4000 to 7FFFh.

	D4  D3

	0    0		Low bank ROM = 0000 to 3FFFh, register page off
	0    1		Low bank ROM = 4000 to 7FFFh, register page off
	1    0		Low bank ROM = 8000 to BFFFh, register page off
	1    1		Low bank ROM = 0000 to 3FFFh, register page on

D2 to D0 determine which of the lower 8 pages of the cartridge ROM appear at the low bank address. The default is page 0.

The logical (as seen by the CPU) to physical (as appears to the upper five cartridge address lines), page translation scheme is thus :

Low bank:	Logical page (RMR2)	Physical page

			0-7		      0-7

High Bank:	Logical page (DFxxh)	        1
		
		0-127 (not disc page)	        1

		0 or 7 (disc page)	        3

		128-255		             0-31

This means that any of the first eight pages of cartridge ROM can be pages to either 0000, 4000, or 8000h, while any of the 32 cartridge pages can simultaneously appear at C000h.

The two ROM disable bits in the existing mode and ROM enable register disable the ROM as before, wherever it is mapped, as does the ROMDIS signal from the expansion bus. The "write through" mechanism, whereby writes to an area which is currently mapped as ROM actually write to the underlying RAM, still functions, wherever the ROM is mapped. However the write through mechanism cannot be used to access the register page. Write through also does not operate to the RAM from the register page.

Analogue paddle ports

The ASIC includes the logic for an octal A/D converter, in conjunction with an external R-2R network, comparator and analogue multiplexer. Eight analogue input channels are thus available on the PCB, of which only four have connectors. This allows support for four paddles or two joysticks, with capacity for twice this many without redesigning the ASIC. The A/D is 6 bits wide, to give sufficient resolution after calibrating joysticks. It appears to the software as a bank of eight, 6 bit, read-only registers from 6808h to 680Fh, known as ADC0-7. They are updated approximately 200 times per second. The A/D inputs have an input range of 0V (data = 00) to 2.5V (data = 3Fh), and an input impedance of 180k to Vcc.

PAL subcarrier locking

The main oscillator for the ASIC is 40MHz. A divide by 9 output at 4.444MHz is provided with a 5:4 mark/space ratio. It is possible to change the main crystal to 9 x 4.33619MHz = 39.90257 MHz, slowing the whole system by 0.25%. This may or may not upset the disk drives, but even if this is the case, a diskless unit could provide PAL subcarrier frequency locked to the master oscillator, thus improving the picture quality.

Locking of enhanced features

The ASIC contains a locking mechanism, whereby the enhanced features are not available until the software has performed an obscure sequence of I/O instructions to the ASIC. This prevents any existing software from having nasty accidents on the new hardware.

The lock is operated by writing a series of bytes to the 6845 address register at address BCxxh. The lock must first be synchronised by writing first a non zero byte value then a zero. The following sequence must then be written:

FF,77,B3,51,A8,D4,62,39,9C,46,2B,15,8A,CD,EE

The lock will then be picked. If it required to lock it again, the same sequence must be followed but without the terminating "EE".

However, it should be noted that unauthorised use of this mechanism may infringe Amstrad's patent .

When the lock is "locked", the secondary ROM mapping register does not exist (see Section 2.6). It is therefore impossible to select (or to deselect) the memory mapped register page.

Eight bit printer support

The ASIC can provide support for eight bit printers. If a link on the PCB is made, the most significant printer port bit will be controlled by bit 3 in register 12 (decimal) of the 6845, i.e. bit 11 of the start address register. If the link is not made, the most significant printer port bit will always be low.

Floppy disc data separator

Because of timescale pressures, the data separator design in the ASIC has been deleted rather than improved . Thus all models with a disk drive use an external SED9420 data separator.

Power requirements

The Arnold V range is a 5V only design. Power requirements are:

Amstrad 464 Plus: MIN MAX UNIT

Main PCB 700 1300 mA

Cassette unit TBD TBD mA

Total consumption TBD TBD mA

Amstrad 6128 Plus: MIN MAX UNIT