Z88 Developers' Notes
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1. Z88 Overview

Processor               Z80 running at 3.2768 Mhz.

Circuitry               Most complex jobs are done by the BLINK gate array

Memory                  The Z88 can address up to 4M of memory. This is
                        achieved by dividing the 64K address space of the Z80
                        processor into 4 16K segments. Each segment can then be
                        assigned to any of 256 16K banks, thus allowing a total
                        of 4M. The first 64 banks are assigned to internal
                        memory and the three card slots at the front of the
                        machine are assigned 64 banks each.

RAM                     32K internal, expandable by addition of external RAM
                        cards. 512K RAM may be soldered as internal RAM
                        replacing the 32K standard (requires also retracking
                        of circuitry).  Current available are 128K, 512K and
                        1MB RAM cards.

ROM                     128K internal, expandable by addition of ROM cards. ROM
                        contains the operating system (OZ), built in
                        applications and hooks for external applications to be
                        integrated into the system. The underlying hardware
                        allows the internal ROM to be expanded to 512K.
                        However, modification of the motherboard is needed to
                        add the extra address lines, since only 128K is

Screen                  640x64 pixels, usually divided into 104x8 6 pixel wide
                        characters and 16x64 pixels of status information at
                        the far right of the screen.

Serial Port             RS232 port, configured as DTE, for connecting a printer
                        or communicating with other devices. Tx, Rx, CTS, DCD
                        and RTS can be independently set and read, both
                        software and hardware handshaking are supported. The
                        serial port runs at the following baud rates: 75, 300,
                        600, 1200, 2400, 9600, 19200, 38400.

Power supply            Internal batteries (4xAA, alkaline) or external 6.5V DC
                        supply. Note NiCad's are not suitable, losing their
                        charge too quickly at the end of their life, thus
                        putting data at risk. Also even if the Z88 is running
                        off the power supply, batteries should be installed.
                        This safeguards against power failure etc, which may
                        cause data to be lost in a machine with no batteries.

EPROM's                 Slot 3 contains circuitry for programming EPROM cards.
                        This circuitry unfortunately means that RAM cards in
                        slot 3 consume much more power than in slots 1 and 2.
                        It is recommended that slot 3 is, therefore, not used
                        for RAM. Note also that when the machine is powered up
                        from cold (eg. for the first time) there is a small
                        chance that an EPROM in slot 3 will be written to and
                        corrupted. Therefore, it is advisable to remove EPROMs
                        if the Z88 has had no power and you are about to apply
                        32K, 128K and 256K EPROM Cards are available.

Software                OZ operating system. Index (application coordinator),
                        Filer (RAM filing system), Panel (for system settings),
                        PrinterEd (for printer settings), Imp Export (for
                        communication with other machines), Terminal (VT 52
                        compatible), Alarm, Clock, Calendar and Calculator.
                        Also three fully blown applications: PipeDream
                        (wordprocessor and spreadsheet), Diary and BASIC (a
                        version of BBC BASICZ80).

Expansion               Three card slots at the front of the machine for RAM
                        or EPROM/ROM Cards. Expansion connector available for
                        OEM applications, but is normally sealed.

Machine States

In order to save power the Z88 is able to close down sections of its circuitry when they are not required for use. As areas of circuitry are closed down the machine is thought of as falling asleep. There are four states of drowsiness which successively use less power. Note that states when the display is on (Awake and Snooze) are often referred to as 'on' and the Coma state is often referred to as 'off'.

Awake       Z80 clock running and display on.

Snooze      Z80 clock stopped and display on, ie. waiting for keyboard

Doze        Z80 clock running and display off, ie. programming EPROM.

Coma        Z80 clock stopped and display off, ie. machine shut down.


There are two forms of reset which can be performed on the Z88. A Hard Reset completely clears the memory, thus any files stored in memory will be lost, and resets all defaults and the real time clock. A Hard Reset is performed by opening the flap, which covers the card slots at the front of the machine, and then pressing the recessed reset button on the left hand side of the unit, just below the power socket. A Soft Reset is performed in the same way, but with the flap firmly closed. A Soft Reset will reset many defaults and remove all applications and alarms. However RAM files should be preserved, though this can never be absolutely guaranteed, and the clock should not be affected. A Soft Reset can also be performed from the Index by using the Purge command.


The Z88 applications are designed so that the user can switch between one application and another, possibly different instantiations of the same application. When an application is interrupted and other applications are instantiated, it is said to have been pre-empted. When the pre-emption is over the application is in the same state as it was before the interruption and can continue as though nothing had happened. Sometimes the application will have to tidy up the screen after a pre-emption, the system will tell the application if this is necessary. Because more than one application can be running at any one time (although only one runs, the others are dormant - the Z88 is task switching not multi-tasking) the resources of the machine must be carefully shared out. This means extra care must be taken in writing applications and observing the rules, since a failure in one application is likely to cause problems for the machine which will effect other applications and stored files.

Menu and help system

The Z88 has a powerful system for entering commands through menu pages. The menu pages can also support a keystroke sequence and a help page for each command. To use this system the application writer simply has to set up a static data structure of the appropriate form in the application ROM. The system will then generate all the menu and help pages and return keyboard codes when a menu or a keystroke sequence is entered. The system can be seen working most fully in PipeDream; ROM space prevented the system having more help pages, but these can be added on external cards.

Devices and files

The input/output scheme of the Z88 is based on named devices. File memory, the screen, the keyboard, the printer and serial port are all treated as devices. To use a device an application attempts to open a channel to the device (with a file name for file devices, otherwise just the device name), and if this is successful a handle is issued to reference that channel. There are a limited number of handles in the system (about 150), and although in practice they are unlikely to run out, the possibility must be taken into account.

The Z88 RAM filing system is divided into 5 devices. :RAM.0 accesses internal memory and :RAM.1, :RAM.2, :RAM.3 access RAM in slots 1,2 and 3 respectively. The fifth RAM device is :RAM.- which can use memory from anywhere in the system. Originally the intention was that this would be used for scratch files which would be lost if a soft reset occurred. Unfortunately a bug in version 2.2/3.0 of the operating system means that if there are any files in :RAM.- when a soft reset (or purge) occurs the system will almost inevitably crash, often requiring a Hard Reset. :RAM.- is best avoided where possible and any files which do use it should be deleted immediately after use. Some international V3.xx and V4 of the OZ operating system has fixed this problem and files may be stored without crashing the machine during a soft reset.

Before listing the available devices note that the EPROM archiving system does not use a device type interface. Also note that it is not possible to add extra devices to the system via application ROMs.

:RAM.n      File device in slot n, 0=internal. Read and write.

:INP.0      Standard input device, usually the keyboard, but can be redirected.
            Read only.

:OUT.0      Standard output device, usually the screen, but can be redirected.
            Write only.

:SCR.0      The screen. Write only.

:COM.0      The serial port. Read and write.

:PRT.0      The printer. Data sent to this device is processed by the printer
            filter whose settings are defined by the PrinterEd application.
            Write only.

:NUL.0      This device does nothing, but can be useful when you want to send
            something nowhere, or get nothing from anywhere. Data sent is
            thrown away, reading returns an End of File message. Read and

:ROM.n      Associated with application in slot n, 0=internal. This device can
            be read using the DOR routines described later, but cannot be used
            as a conventional device.

Streams and Filters

A stream is simply a communication channel associated with a handle. Various system calls can take input or send output to a stream. For example the system routine to convert binary to ASCII decimal numbers can direct its output to the standard output stream rather than store the result in a buffer in memory and then output the contents of that buffer as a string. A filter is a general way of performing simple transformations on character sequences. Text can be sent to a filter and then pulled out again, having been processed according to the set of transformations associated with the filter. Some system routines take a filter as their input, requiring the application to have already sent text to the filter, and others as output, requiring the filter to be read after the routine is complete.


The Z88 uses interrupts extensively in its general operation. Unfortunately for external hardware it is not possible for the system to recognize and deal with interrupts from outside sources - it is uncertain what the system would do with an external interrupt. The following are the sources of maskable interrupts, which are all mediated by the BLINK chip:

Keyboard    A key press causes an interrupt which can wake the machine from a

Flap        This, if the machine is on, will switch it into Doze, in
            preparation for card insertion and removal. Note that the flap
            should only be used when the machine is on, otherwise its movement
            is not properly recognized.

UART        The serial processor in the BLINK chip generates interrupts to
            coordinate communications.

Battery Low This causes OZ to display a warning message in the status area (or
            OZ window) on the right hand edge of the screen.

Clock       The clock interrupts periodically so that the Z88 can update the
            time and date and check any pending alarms.
Non-maskable interrupts come from two sources, and again they are mediated by BLINK:
Power fail  This puts the machine in a coma when power is very low.

Timeout     When the machine has been inactive, but on, for the timeout period
            set in the panel, this interrupt occurs and puts the machine in
            coma. Note certain activities reset the timeout value, such as
            pressing a key, or reading the clock. This latter is why the
            machine will not timeout when the Clock is running.

Expanded and Non-Expanded Machines

Depending on the amount of memory in slot 1 the Z88 will either be expanded or non-expanded. With 128K or more of RAM in slot 1 the Z88 is said to be expanded. This has the effect of increasing the maximum width of the graphics map to 256 pixels from 96, increasing the default bad application size, used by BASIC, from 8K to 40K, and expanding the number of user definable characters from 16 to 64. If extra memory is required, but not these expanded parameters, extra RAM should be placed in slot 2 (or less preferably in slot 3).

V4 of the OZ operating system automatically recognises the machine as expanded if the Z88 has been upgraded with 128K or more of internal RAM (replacing the original 32K RAM). However this requires soldering.

The Screen

The screen is a supertwist liquid crystal display, driven by the BLINK chip, and capable of displaying 640x64 pixels. This is usually arranged as 104x8 6x8 pixel characters and 16x64 bits of status information on the right of the screen (the OZ window). However an area of 10x8 characters on the left hand edge of the screen is reserved by the system for the application name and menu topic names. This gives applications a working area of 94x8 characters.

The operating system provides a screen driver, which operates by sending special codes to the screen, which provide a number of useful display facilities: six windows (with optional borders and banners) for application usage; up to 64 user defined characters; optional centering and left or right aligning; tiny, bold, grey, inverted or flashing characters; horizontal and vertical scrolling; controllable bleeps; the greying and ungreying of windows; access to extra characters (such as box drawing, arrows and keyboard icons). The driver provides access to all the display facilities of the machine except high resolution graphics. High resolution graphics are generally provided by the map interface, so called because PipeDream uses it to make a map of the page being worked on. This is 64 pixels deep and is from 8 to 256 pixels wide in 8 pixel steps. Note that the maximum width is 96 pixels if the Z88 has less than 128K RAM in slot 1, ie. is unexpanded. The interface allows only for a whole line of pixels to be written at a time and there is no facility for reading back from the screen, so if necessary the application needs to keep track of what has been written. It is possible to access the screen more directly, although this is complicated and may interfere with other applications, so where possible the map interface is to be preferred.

The Z88 Assembler Workbench provides library routines for plotting points, drawing lines and sprites, and many other graphics utilities for in the graphics area. The library routines are properly interfaced with the Z88 operating system, and can easily be integrated into applications.

Card Manager

The card slots at the front of the machine are the route by which memory can be added and external software can be provided. EPROM cards, used for archiving, are not part of the card manager scheme, since they do not effect any part of the system outside of the Filer. RAM and ROM cards have effects throughout the system and so the card manager exists to keep track of their movements. The most basic rule of the card manager is the machine must be on (ie. the display lit up) and the computer in the Index popdown. At this stage RAM and ROM cards may be freely added. It is the removal of cards, however, which is problematical.

Removing a RAM card is a dangerous business. If a card is removed, a flashing FAIL message will appear at the top of the OZ window and the Z88 will not respond to the keyboard. The machine should now be soft reset (replacing the RAM card will not un-fail the machine because the data on the card will have been lost), and with luck files in other RAM devices will be intact. This cannot be guaranteed and it is recommended that files are backed up on EPROM or on another computer. Fortunately removing a RAM card need only be done in exceptional circumstances, such as upgrading to a larger RAM. If a FAIL message occurs without removing a RAM card, then some of the RAM memory in the machine may be faulty, however a software crash can also cause a FAIL state to occur.

ROM cards should only be removed from the Index, with the machine on. Attempts to remove a card when not in the Index will result in a continuous high pitched tone and a flashing INDEX symbol. The system will be locked until the card is replaced in its original slot. After replacement go to the Index before attempting to remove the card. If an application on the ROM card is being used, indicated by the slot number next to the application name in the suspended activities list, and the card is removed, the system, once the flap is reclosed, will bleep and flash CARD in the OZ window. The card must be replaced before anything else can be done. Removing cards is very straightforward providing you make sure the machine is switched on and close the flap after each removal and insertion, to check that no rules have been disobeyed.

Alarms, timeouts and batteries

If an alarm occurs when the machine is in coma, the machine will be woken up and held in lock out mode. This is indicated by a flashing LOCK-OUT symbol in the OZ window. Lock out means that the keyboard is disabled and normal working must be restored by switching the machine off and on again using the two shift keys. The idea behind lock out is that an alarm may awake the Z88 in a brief case, or some such, and lock out means that any keys which may accidently be pressed will have no effect. The machine will remain awake until the user intervenes or the machine times out. Lock out mode can be forced by holding down CAPS LOCK before releasing the two shift keys when switching the machine on.

The timeout mechanism causes the machine to enter coma if it is on, but inactive for the time set in the panel. The period is set in minutes from 0 to 99 where a value of zero means never timeout. Keypresses and reading the clock both reset the timeout count, meaning, for example, that if the Z88 is in the Clock page it will never timeout. The timeout mechanism can be used to save power, by ensuring an unattended machine switches off after a short period. It may also be useful to have the machine permanently awake, if waiting for input from the serial port, for example.

When battery power is getting low a battery low interrupt occurs and a BATT-LOW symbol is displayed in the OZ window. This is a warning that batteries should be replaced in the near future. If batteries are not replaced, and the system is in coma, the memory will be safe for days or even weeks. If machine usage continues while the BATT-LOW symbol is present then eventually it will begin to power fail. This is marked by violent screen flickering and the machine going into coma. It becomes increasingly difficult to awake the machine. At this point memory integrity may be in danger and fresh batteries should be installed immediately, preferably a power supply should be attached during this operation. If possible batteries should always be replaced before a power fail can occur. Do not continue to try and work with the machine after a power fail has taken place.

Nickel Cadmium rechargeable batteries supply a steady current until near the end of their charge, at which point the power they provide falls very sharply over a short period of time. Alkaline batteries lose their power slowly and so as their charge falls the battery low warning can be given while there is still enough charge to keep memory safe. With NiCad batteries there is no warning as to when they will lose their charge, the first the user is likely to know about it is a dramatic power fail. The consequence of this is that data loss is very likely when NiCads reach the end of their charge, therefore they are not recommended for use with the machine but may be used with great attention.

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Introduction Z88 Overview Application Writing