Torch Z-80 Acorn Z-80 and 6502

To the BBC by 

bus and Tube 

Roger Cullis explores some second-processor options for the BBC Micro. 

ALTHOUGH the BBC Micro is a very fast 

machine with comprehensive facilities for 

driving peripheral devices, it was planned 

that it would ultimately be the basis of a 

much more powerful system. 

The 6502 which lies at the heart of the 

BBC Micro is an eight-bit microprocessor, 

capable of addressing only 64K of memory. 

Of this, the powerful machine operating 

system and memory-mapped I/O takes 16K 

of ROM and a further 2.5K for zero page 

storage, processor stack and I/O buffers. 

The Basic interpreter requires 16K and the 

screen buffer needs up to 20K. As a 

consequence, there may be less than 10K 

available for user programs. Acorn's answer 

to this limitation has been to dedicate the 

basic computer exclusively to input/output 

and other housekeeping functions and to 

provide a second processor for data 

manipulation. 

Ignoring the RS-432 port, which is too 

slow for this purpose, there are two practical 

ways of connecting a parasite processor: via 

the 1MHz bus or via the Tube interface. The 

latter provides faster communications, since 

it runs at 2MHz, but at the same time is less 

tolerant of timing inaccuracies. Most second 

processors are Tube based, a notable 

exception being the Torch Graduate. 

Torch Z-80 

The first commercially available second 

processor for the BBC Micro was the Torch 

Z-80, which was launched as a component 

of the Torch Z-80 disc pack in September 

1982. Apart from a change to half-height 

drives, the current pack is mechanically 

similar to the original product and 

comprises a metal housing containing 28track 

disc drives and a heavy-duty power 

supply. A ribbon cable couples the floppies 

to the disc interface connector on the 

underside of the BBC Micro. 

The second processor electronics are on a 

double-sided printed circuit board which is 

mounted inside the main computer. The 

board contains a Z-80A running at 4MHz, 

64K of dynamic RAM, a 2732 bootstrap 

EPROM and a 6522 versatile interface 

adaptor which handles communications with 

the base processor in the BBC Micro. 

A short 40-way ribbon cable from the Z-80 

printed circuit board passes through a slot in 

the BBC Micro's case and plugs into the 

Tube connector on the underside. A 

sideways ROM which contains the BBC 

Micro housekeeping routines completes the 

setup. 

Torch has done a neat job with the 

sideways ROM, which has evolved from 8K 

to 16K to provide system enhancements. 

The current version, MCP 0.41, contains the 

routines necessary to interface all the Torch 

second processors, other than the Graduate, 

as well as the firmware to control the 

Torchnet local area network. The operating 

system for the Z-80 is CPN 0.71, which is 

configured in the same way as CP/M 2.2 

and permits the user to run CP/M software. 

The advantages of having the operating 

system in ROM are that the cold boot is 

instantaneous, 63K of RAM is available for 

user programs and system tracks are not 

required on the disc. CP/M utilities such as 

Format, Copy, Input, etc. are normally 

loaded in from disc. With CPN 

The Torch Z-80 pcb fits under the lid of the host micro. 

they are in ROM and instantly ready. BBC * 

commands are also available from ROM. 

Torchnet, which will operate only if the 

BBC Econet hardware is present, is an 

enhanced version of Econet which does not 

require dedicated stations for file and printer 

servers. Bundled with the Torch disc pack is 

the Z-80 version of BBC Basic as well as 

Perfect Writer, Perfect Speller, Perfect Filer 

and Perfect Calc. 

Acorn Z-80 and 

6502 

In fact, Z-80 and 6502 are misnomers, 

since the processors are actually a Z-80B 

and a 65CO2. The Z-80B is simply a faster 

version of the Z-80, but the 65CO2 also 

enhances the instruction set of its 

predecessor. There are also some timing 

changes and the famous 6502 pagespanning 

bug has been removed, together 

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with the illegal instructions which some 

people have used as a means of program 

protection. 

Conceptually, the two second processors 

are very similar. Both are mounted in a 

narrow case styled to match that of the BBC 

Micro, both have a power supply with 

toroidal transformer and transistor regulator 

on the main circuit board, both address 64K 

of RAM with a 2732 bootstrap EPROM and 

both communicate with the Tube through a 

Ferranti ULA chip. 

Apart from the differences which are the 

result of the different processors, the main 

distinguishing feature is the software which 

is supplied with the Z-80. To say that there 

is an abundance is an understatement; a 

separate box is necessary to hold the 11 

manuals and seven discs. Clearly Acorn was 

not certain of its target market, so it took a 

shotgun and aimed at everything in sight. 

Surprisingly, Acorn chose to make CP/M 

2.2 the operating system for the second 

processor even though the much more userfriendly 

CP/M Plus has been available for 

over a year. A possible explanation is that 

CP/M Plus requires an extra 64K bank of 

memory to exploit it fully. With the current 

price of chips it would not have added 

greatly to the cost, and it would have been a 

real advantage. 

A much more serious drawback is that the 

accompanying manual is no more than the 

infamous Digital Research CP/M 2.2 

manual in a BBC Micro cover. My advice to 

new users is to throw it away — having 

preserved Appendix J, which contains the 

BBC-specific details — and buy one of the 

really good books on CP/M, such as CP/M 

and the Personal Computer by Dwyer and 

Critchfield or CP/M Primer by Murtha and 

Waite. 

One particularly interesting aspect of this 

implementation, is that it includes GSX 

graphics. GSX-80 is an attempt by Digital 

Research to establish standards for the 

software control of graphics devices such as 

printers and plotters, tracers and light-pens. 

It is based on a graphics operating system 

with a structure like that of CP/M itself. Just 

as CP/M has a Basic disc operating system 

which interfaces the user program to the 

physical device drivers of the basic I/O 

system, so GSX has a graphics device 

operating system which links the user 

program to the real devices through the 

graphics I/O system. Physical device drivers 

are provided for the VDU screen, modes 0 

and 1, and for the Epson and Microline 

printers. 

As well as a Z-80 version of BBC Basic 

written by M-Tec, Professional Basic is also 

supplied for applications which require a 

Microsoft-style Basic. Cobol users will be 

pleased to see that Acorn has included the 

industry-standard Cis Cobol written by 

Microfocus, as well as the two major 

Microfocus Cobol tools, Animator and 

Forms-2. 

Acorn's Z•80 (left) and 6502 are housed in separate matching boxes. 

Moving up through the packages, the next 

level is a program generator, Nucleus, 

which can be used by those with no previous 

programming experience since it is menudriven. 

It can create databases, such as 

records of club members, prepare planning 

schedules, print reports from accounting 

programs or design reports to be prepared 

from programs written by Nucleus itself. 

There are three office software packages: 

Fileplan, described as a database 

management program or an automated card 

index; Graphplan, a financial modelling 

spreadsheet; and Memoplan, a simple word 

processor. Complementing these is 

Accountant, which provides simple bookkeeping 

facilities. 

Acorn has already established a large user 

base among small businesses, which will 

find these packages useful and adequate for 

many of their needs. However, apart from 

CP/M and Basic, which are essential 

components of any system, the other 

packages will only be of value to a limited 

proportion of potential users. The others 

would almost certainly welcome the 

opportunity to purchase the second 

processor without paying the £200 premium 

for the extra software. 

Compared with the Z-80, the 6502 second 

processor is naked and is intended for 

people who have run into limitations, either 

of memory size or speed, when running 

software on the unexpanded BBC Micro. 

Apart from the DNFS sideways ROM, the 

only software supplied with the 6502 

processor is ROM-based HiBasic, which is 

BBC Basic assembled at a higher address to 

give the user a larger amount of free 

program area when running BBC Basic 

programs in the second processor. 

At present there is not a lot of software 

which has been specifically written for the 

6502 second processor. Bitstick needs it, as 

does the Level-2 file server software for 

Econet. Acornsoft has prepared a special 

version of the View word processor, and a 

comprehensive macro assembler package, 

the 6502 Development System, is in the 

pipeline. This will produce code for the 

6502 or the 65CO2. 

When the BBC Micro was introduced, 

Acorn issued dire warnings about not 

accessing the operating system and 

peripherals directly. All use of the MOS 

routines should be via specified calls such as 

Osbyte or Osword. Those who wrote 

directly to the I/O devices or made Pokes to 

the screen are now finding that their 

software is having to be rewritten to run on 

the 6502 second processor. This is true of a 

number of sideways ROMs as well as most 

arcade games which include animation 

techniques. 

Both Acorn second processors expect OS 

1.0 or higher and are provided with a 

voucher for free exchange of earlier 

versions. They also come with a 16K DNFS 

filing system ROM which replaces the 8K 

disc filing system and 8K Econet filing 

system ROMs, and also contains the 

interface routines to initialise the Tube for 

second processor communication. 

CMS 6809 

The most exciting of the eight-bit add-ons 

is the 6809 system from Cambridge 

Microcomputer Systems. It is an engineers' 

system based on the microprocessor that 

arrived too late. Compared with the 6502 

and Z-80, the Motorola 6809 has a whole 

orchestra of bells and whistles. Had it been 

developed sooner it would have been 

adopted for a wide range of computers 

rather than just the Dragon and the Tandy 

Color Computer. 

The Motorola 6809 has two eight-bit 

accumulators, A and B, which can be

CMS mini-rack development system. 

combined as a single 16-bit accumulator, D. 

It has two 16-bit index registers, X and Y; a 

16-bit user stack register, U, which is a cross 

between a typical microcomputer index 

register and a typical microcomputer stack 

pointer; a 16-bit hardware stack pointer, S; a 

16-bit program counter, PC; an eight-bit 

direct page register, DP; and an eight-bit 

condition code register, CC. 

The direct page register holds the eight 

most significant bits of an address and 

extends the concept of zero-page addressing 

to any page in memory. It thus allows the 

programmer to take advantage of short-paged 

addressing without being limited to the first 

256 bytes of memory. Different programs 

can have different base pages, eliminating the 

need for multiple use of page zero locations 

and reducing the possibility of conflict. 

CMS sells a 6809 Eurocard which you 

CMS colour-graphics card (left) and 6809 

card. 

can mount inside the case of the BBC Micro 

in the same way as the Torch Z-80 card, but 

its full potential is only realised as the engine 

which drives the CMS version of 

microprocessor Meccano, a rack-based 

development system connected to the BBC 

Micro by a trailing umbilical ribbon cable. 

Unlike the other second processors in this 

review, whose main purpose is data 

processing, the CMS 6809 is mainly used to 

develop systems which will be used in 

control applications. The heart of the system 

is a neat 8in. rack containing a switchedmode 

power supply and a seven-way 

buffered backplane which can be used to 

connect a variety of Eurocard-based subsystems. 

As well as the 6809, the processor 

card itself carries 64K of dynamic RAM, a 

4K bootstrap EPROM and a pair of back-toback 

6522 VIAs which communicate with the 

Tube by way of a 40-way IDC connector. 

Situated at the other end of the card is a 64way 

DIN 41612 connector which plugs into 

the backplane. 

Among the other CMS cards which also 

plug into the backplane are a highperformance 

colour graphics card, an IEEE 

talker/listener controller, a universal 

controller with either a 6809 or 6502 

processor, a memory carrier and a variety of 

analogue and digital interfaces. 

The system is robust. Mechanically it is 

soundly made and the Tube connector is 

three or four times as long as those which 

operate the other second processors without 

problems of noise or crosstalk. It is even 

possible to insert and unplug the cards with 

the system powered up and only experience 

the occasional crash which requires no more 

than a reboot to correct. 

The colour graphics card is based on the 

Thompson EF-9366 graphics processor chip 

and has a 64K graphics memory compared 

with 20K in the highest-resolution mode on 

the BBC Micro. It is capable of displaying 

mixed text of differing sizes, orientations and 

colours as well as pictorial data. It requires 

only 32 bytes of system memory. 

The colour display has three primary colour 

planes and a luminance plane with a flashing 

option. Each colour plane is 512 pixels wide 

by 256 pixels high and is capable of drawing 

1.5 million dots per second. In monochrome 

there are 16 grey levels, and in colour eight 

colours per pixel with a separate intensity 

plane. The full ASCII character set is held in 

ROM and a high-density text mode supports 

32 85-character rows in any colour 

combination. 

As this processor is aimed at the industral 

market, the only software supplied with it is a 

sideways ROM to initialise the BBC Micro 

and get the Flex operating system up and 

running on the 6809. There are a large 

number of software packages available which 

will be of immense value to the software 

development engineer both for the 

development of 6809 software and for work 

with other processors. Reflecting the nature 

of the CMS system as a development tool 

for control applications, many of the software 

utilities produce code which can easily be put 

into ROM. One of these is the 

editor/compiler/tracer, PL/9. It is a structured 

language, having procedures and statements 

such as IfThen-Else, which the user of BBC 

Basic will find familiar. Lucidata Pascal is 

another high-level language whose programs 

can be saved to ROM. Among the low-level 

utilities is a range of cross-assemblers, 

disassemblers and simulators for the 6809, Z- 

80, 6502, 6800, 8080/8085 and 68000. 

CMS 6502 

A very recent introduction is the CMS 6502 

second processor. Based on the CMS 

universal controller card, it permits the user 

to select a variety of combinations of ROM 

and RAM up to a total of 160K. Address 

decoding is by means of a bipolar ROM, 

giving the opportunity to redefine the 

memory map. Provision is made on the 

printed circuit board for a real-time clock and 

battery backup, which are available as an 

optional extra. 

As with the 6809 card, it may be mounted 

as a stand-alone board inside the case of the 

BBC Micro, or it may be plugged into the 

rack-based bus system for further expansion. 

The card has a 4K monitor ROM which 

contains the routines necessary for Tube 

communications and will run all base 

processor software which is compatible with 

a 6502 second processor. 

CMS 6502 card. 

Xcalibur Xmem 

Another company looking to the industrial 

market is Xcalibur with its Xmem, which 

seeks to marry BBC technology to some of 

the vast range of peripheral cards that have 

heen developed for the Apple II. Unlike the 

other products included in this review, the 

Xmem is not a second processor. It does not 

contain a processor chip at all; instead, it 

provides a 64K bank of expansion memory 

coupled with five standard Apple expansion 

slots into which Apple cards can be plugged. 

The Xmem is connected to the 1MHz 

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bus and is accessed by calls to Jim, page FD 

extended memory, the pages being selected 

one at a time, using the Fred paging register. 

Power is obtained either from the auxiliary 

power-connector socket on the BBC Micro or 

from an independent power supply. 

There are a number of limitations on the 

connection of peripherals, mainly stemming 

from the passive nature of the device and its 

alien environment. Since the Xmem has no 

6502 processor and no Apple monitor, there is 

no means of controlling Apple cards with 

routines in ROM. But this does not present a 

limitation to Xcalibur, which produces a 

range of accessories for industrial 

applications. 

Typical of these is the Xad-1 four-channel 

12-bit analogue-to-digital converter with a 

battery-backed real-time calendar/clock. It 

also controls four channels of isolated relay 

outputs. The analogue input channels are set 

to a nominal OV to 5V range and so can be 

used to monitor TTL digital lines or contact 

closures, as well as continuously variable 

analogue levels. The relay contacts are 

individually programmable and may be used 

to actuate external devices, so the card may 

be used as the basis of a process control 

system. 

Among other cards available are larger 

and smaller analogue-to-digital converters, 

thermocouple amplifier modules, counters 

and digital-to-analogue converters. The 

Xmem card itself will fit into the standard 

Xcalibur portable computer case. This will 

also take a BBC motherboard and the 

Xcalibur 5in. monitor so that a transportable 

industrial control system can be assembled. 

Xcalibur Xmen with I/O converter and 

clock card. 

Torch Graduate 

"Add the capability of running IBM software 

for less than £1,000" was the offer made by 

Torch when it launched the Graduate. There 

are four versions, ranging from a singlefloppy 

128K machine to a twin-floppy 256K 

RAM model which offers almost complete 

IBMulation. Using BBC graphics, a 40column 

full-colour display can be 

The Torch graduate outside ... 

... and inside 

obtained, which can be extended to 80column 

mode by plugging in a standard 

IBM-compatible expansion card. An 80column 

monochrome display is achieved via 

the BBC display. 

The Graduate is an exception among BBC 

Micro second processors as it does not use 

the Tube to connect its 8088 to the 6502 

base processor. Instead, communication is 

through the 1MHz bus. Another feature is 

that it does not require the installation of a 

disc filing system in the host. Its floppy 

discs are completely self-contained. Two 

expansion slots are provided for addition of 

standard IBM expander cards. 

Torch Unicorn 

By far the most powerful second processor 

currently available for the BBC 

Micro is the Torch Unicorn HDP-68K, 

which comes with a Motorola 68000 16/32bit 

processor and a 20Mbyte hard disc and a 

400K floppy as backing store. As supplied, 

there is 256K of dynamic RAM but by 

changing internal links and substituting 

256K chips, this can be 

The Unicorn's motherboard is mounted vertically beside the drives.

increased to one megabyte. The main circuit 

board which carries the processor chip and 

RAM is mounted at one side; and auxiliary 

cards are provided for the hard-disc 

controller and SASI disc interface. A 100W 

switched-mode power supply and a cooling 

fan are also squeezed into the case, which 

measures only 190mm. by 270mm. by 

330mm. 

The operating system for the 68000 

Unicorn is Unix III with the Berkley 

enhancements. It comes on 24 floppy discs 

which contain over 200 programs and 

utilities. There is a full set of supporting 

documentation including the standard Unix 

manuals and a description of the Torch 

enhancements. The complete Unix pack 

contains a C compiler and editor, the vi 

screen editor and the csh C shell. Languages 

available include Fortran 77 and Pascal. 

To enable the user to run existing CP/M 

software as well as Unix, the parasite 

processor card also carries a Z-80B. Due to 

the faster memory chips required by the 

68000, the Z-80B can be run at 6MHz, 

rather than the 4MHz of a normal Z-80 

second processor. This means that CP/M 

programs will run 50 percent faster. A 

The Torch Unicorn. 

memory management unit allows multitasking 

without interference between tasks. 

Conclusions 

• With the second processors covered by 

this review, BBC Micro owners have the 

opportunity to enhance their systems to 

virtually any degree within the current 

state of the art. 

•The availability of the CP/M, Flex, 

UCSD p-system, Xenix and Unix 

operating systems together with languages 

such as Basic in many forms, Pascal, 

Cobol, Forth, Lisp and Fortran mean that 

there is the capability of running 

practically all serious software available 

for microcomputers. 

• It is time for manufacturers to review 

the practice of giving away software with 

new machines. Most buyers either have a 

specific application in mind or have 

already invested a large amount of time 

and effort in installing their programs on 

another machine. They would be far better 

served by a service which upgrades and 

configures existing software for the new 

machine. µ 

PRACTICAL COMPUTING December 1984 85

Torch Z-80 Acorn Z-80 and 6502

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