CONTENTS

Introduction - Program Description - Registration - Compatibility - Comparison - Starting XTender - AutoLoad-n-Run - Commandline Qualifiers - Screen Output - Keyboard Facilities - Keyboard Compatibility - Mouse Control - File Management - Printer Output - High Resolution - ZX81 64k Memory Map - ROM and 8-16k Area - Stopping XTender - Clock Frequency - Slowing Down - Z80 Emulation - Programs from Tape to Disk - Screen Saver - Warranty - About the Author - XTender Release History & Future - Final Notes
 

INTRODUCTION

Although the ZX80 was quite a success (50,000 sold), it has some shortcomings, e.g. it can only do integer calculations (no floating point), the screen flashes at every keepers and goes blank during execution of programs (this does increase speed but is not very 'ergonomic'). An upgraded version of the ZX80 appeared within a year. It has an 8k ROM (instead of the 4k of the ZX80), can do floating point mathematics and offers a choice between FAST operation with black screen (as on the ZX80) or SLOWer operation with normal screen. This updated version is called ZX81.

The ZX81 has a Zilog Z80A processor operating at approx. 3.2 MHz, 1k RAM (yes!, only 1024 bytes, but expandable to 48k), 32x24 text screen and 64x48 graphics resolution in black and white. Programs can be stored on tape and be retrieved at 300 baud. The first versions sold for 70 pounds (ready built) and 50 pounds (DIY kit); 50,000 ZX81s were purchased in the first 3 months after its release.

Later computers from Sinclair include the ZX Spectrum (1982, Zilog Z80 processor at 3.5 MHz, 16k ROM, up to 48k RAM, colour, sound, high resolution, a lot faster thanks to video hardware instead of -software, ASCII based character set and more expansion possibilities) and the QL ("Quantum Leap", 1984, Motorola 68008 processor at 8 MHz, 48k ROM, 128k RAM, multitasking and superb operating system Qdos).

Although the ZX Spectrum undoubtedly was (and still is) the most popular of all Sinclair machines, I think that many people using PC's nowadays may have been introduced to the world of computers by way of their (late) ZX81. I hope these people will re-enjoy working with this simple yet very fine computer again.

Run XTender and be my guest!
 

PROGRAM DESCRIPTION

REGISTRATION

Test XTender during a trial period of 21 days and then delete the files if you don't like to use the program.

However, if you do like to use XTender, please express your appreciation and support my efforts by becoming a registered user. This will entitle you to the following benefits:

• You will receive the complete registered package containing the most recent version of the emulator and 3 library volumes, containing a total of 240 original ZX81 programs. The registered copy of the emulator offers various features which are not present in the shareware version. Most importantly: only registered copies can be slowed down! The registered package also contains the ZX81tape tool, to convert your own ZX81 tapes to PC-files.
• You will be entitled to free technical support by the author.
• You will receive the first official release of XT2 (the succesor of XTender), once available.

A registered copy of XTender, including three library volumes and the services described above is priced at US $ 20.

Orders can be placed via PayPal only. PayPal is the leading provider of world-wide secure internet transactions for eBay and many other sellers, both professional and private. Membership is fully free, and the payments you make are taken off your bank account or credit card account in your own currency. No additional fees are charged. If you want more information about PayPal, click here.

To order your registered copy of XTender instantly via PayPal, just click this button:

Once your order has been received, a registered copy of XTender will be sent to you by email.
 

COMPATIBILITY

Apart from these exceptional applications, any program running on the ZX81 should also run on XTender.
 

COMPARISON

• Safe operation in a well-stabilized PC: none of the tricky business with wobbling RAM packs!
• Full 64k RAM available, including POKEable ROM and 8-16k area!
• Reliable and fast data-storage: no more slow tapes!
• The ZX81 display file can be moved to any part of memory without a crash, machine code can also be executed in the top 32k of memory: on the ZX81 only the bottom 32k could be used!
• A choice of screen colour, and a screen invert option.
• SuperBreak facility.
• Reset under keyboard control.
• etc. etc. etc.

STARTING XTENDER

Two versions of XTender exist. These are named XTENDER! (with exclamation mark) and XTENDER (without exclamation mark). The former is the fastest version and should be used on slow machines (XT, AT). The latter is not speed-optimized, hence a little slower. However, this one has the possibility to slow-down emulation considerably and should be used on (very) fast machines; this slow version is supplied with registered copies of XTender only.

To start the program, simply type

(whichever you prefer or have) at a DOS prompt or double-click in your Windows file browser. This will start XTender. First you will see a title page which displays some general information about XTender and its usage (Note: in case of registered copies, the title page is skipped, use the /t qualifier to see it, see section Commandline Qualifiers). Now press any key to start the actual ZX81 emulation. Depending on the speed of your computer, you will see some flashes and/or weird characters while the ZX81 performs a reset, but eventually the famous inverse-K cursor should appear and XTender is ready to be used.

XTender tries to determine what video card you have installed simply by looking at the current video mode so as to make a distinction between Hercules and non-Hercules. In Hercules, only a small portion of the display will be used, in all other cases, the ZX81 screen occupies almost the entire display. If you have CGA emulation on your Hercules card and want to use it with XTender, make sure you do not start XTender in video mode 7. Alternatively, you can force Hercules or CGA mode by the /e and /c qualifiers, respectively (see section Commandline Qualifiers).
 

AUTOLOAD-n-RUN

This will start XTender as usual, but instead of showing the K-cursor after reset, it will auto-load the ZX81 program "program.p". If this is a self-running program, the program will even start running automatically after auto-load. Otherwise, you will get the K-cursor as usual (but ofcourse the program you specified is present in ZX81 memory!). If the file cannot be opened, the ZX81 will reset and start as usual (no error is reported).

AutoLoad-n-Run will work only once. After NEW or RST-0 or CTRL-ALT-ESC, the program will not be auto-loaded again.

Note that the filename supplied in the commandline must be a DOS-type filename ("8.3") without a path (unless the total size of path and filename does not exceed 12 characters: everyhing beyond the 12th character is ignored). The extension ".p" must always be typed. If you supply more than one filename, only the last one will be used. Qualifiers may be placed in front or behind the AutoLoad-n-Run filename.

If the P-file specified cannot be opened, this is reported and XTender will not start.

For DOS-type filenames, you can even AutoLoad-n-Run ZX81 programs from your Windows file browser by linking the file-extension ".P" to the XTender-executable.
 

COMMANDLINE QUALIFIERS

/?
display help page, briefly summarizing all qualifiers.

/b:nn
set background colour; this option only works if you have an EGA/VGA video card; nn is a decimal number in the range 0 through 15, corresponding with the 16 colours in the regular EGA palette.

/c
force CGA screen mode; use this qualifier when XTender tries to use Hercules although you have a CGA (or better) video card installed.

/d:nnn
registered copies only: set emulator delay factor; nnn is a decimal number in the range 0 through 255; this qualifier is equivalent to the ZX81 command POKE 23,nnn after XTender has been started; for more information, see section Slowing Down. XTender! (with exclamation mark) cannot be slowed down (qualifier not available).

/e
force Hercules screen mode; use this qualifier when XTender tries to use CGA although you have a Hercules video card installed.

/f:nn
set foreground colour; this option only works if you have an EGA/VGA video card; nn is a decimal number in the range 0 through 15, corresponding with the 16 colours in the regular EGA palette.

/h
display help page, briefly summarizing all qualifiers.

/i
force inverse video (e.g. white characters on black background), which seems more comfortable for the eyes in case of serious applications. Note that keystroke F2 or CTRL-F2 can be used to toggle screen colour once the emulator is started (see section Keyboard Facilities).

/k
alternative keyboard circuit. Some programs, especially games, may fail to respond to the keyboard. If this happens, these programs may have been designed specifically for a TS1000 (NTSC version of ZX81); use the /k qualifier to emulate this properly (but a cure is not guaranteed).

/m
skip ZX81 memory test; after a NEW or reset, the ZX81 ROM performs two main actions: it tests the RAM and sets up the display file. Each of these takes about half the reset-time. On an emulator, the memory test is not necessary and can be omitted by this qualifier, thus reducing the reset-time by at least 50%. Note that the RAM is still cleared (zero-filled).

/r:nn
set default ZX81 RAM size. On the original ZX81, the RAM size after reset is always 16k (or less if less RAM is available), because this number is fixed in the ROM. On the emulator, you may change this number to get any RAM size after a reset from 1k up to 48k (set nn to a value of 1 through 48).

/s
disable screen-saver (registered copies only).

/t
show title page of registered copy. On registered copies, the title page is skipped by default. In case you are a registered user and wish to see the title page (containing a brief description of key usage and your name), use this qualifier.

/x
enable mouse controls. By default, mouse controls are disabled.

Qualifiers may be placed in front or behind the autoload-n-run filename.
 

SCREEN OUTPUT

Before XTender starts emulating, it first determines the relative speed of your computer. It does this by running a benchmark test and the resulting benchmark is used to calculate the rate of screen update. For example, if the measured benchmark is 1000 (approx. 33 MHz 386dx machine), the screen is updated at a rate of 100 Hz. Lower/higher benchmarks result in proportionally lower/higher screen update frequencies. As a result of this method, the performance of XTender is automatically optimized for any machine. I have tested XTender on an original PC (benchmark 50, screen updated at 5 Hz) and the emulator (fast version) still executed at a speed of 35% with respect to the real ZX81!

You can see the measured benchmark and resulting screen update frequency by loading the program SYSINFO.P directly after having started XTender. This will provide you with the information you need.

The timer interrupt of the PC can be set to one single frequency only. However, XTender actually needs three distinctively different frequencies:

• 18.2 Hz to keep your PC's internal clock synchronized (you'd be cross with me if I hadn't done this!).
• 50.0 Hz to update the ZX81 system variable FRAMES (for processor-speed-independent timing loops).
• xxxx Hz to update the PC video RAM (xxxx is an arbitrary number determined by the measured benchmark).

An XTender user pointed out that (some?) screen-grabbers seem to work fine with XTender, so if you want to make GIFs (or whatever graphical format you prefer) of your favourite ZX81 pictures, a screen grabber may be a useful tool to do just that!
 

KEYBOARD FACILITIES

As an example of keyboard operation, take a look at the A-key on the help screen. It contains the following symbols:

• the letter A itself
• the command NEW
• the command STOP
• the function ARCSIN (abbreviated ASN)
• a grey block

The K-cursor (for Keyword) always appears at the beginning of a command line and after a THEN keyword; the L-cursor (for Letter) appears elsewhere. The F and G-cursors (for Function and Graphics) can be activated by pressing Shift-Enter (or keypad-5) and Shift-9, respectively.

The default ZX81 screen colour is black characters on white background (unless you have used any one of the /i, /b or /f qualifiers, see Commandline Qualifiers). For games with a lot of graphics this may be OK, but for text handling, I find the inverse far more comfortable for the eyes. Press F2 or CTRL-F2 to change the display from black on white to white on black and vice versa. Alternatively, you may wish to use F4 and/or F5 to change the fore- and background colours (EGA and VGA only): a regular 16-colour EGA palette is available.

Some ZX81 (machine coded) programs do not allow you to stop them using Break (Space key). On XTender, programs can always be stopped using CTRL-ESC. This will actually cause a jump to an address in ROM where a RST 8 for error D is located, and hence the stack is cleared, the error message is displayed and you are back in Basic. Some programs crash the ZX81 if you try this, as they may have messed around with the system variables, Basic area, screen or even with the ROM code. If CTRL-ESC causes a ZX81 hang-up, press CTRL-ALT-ESC to recover. This restores the original ROM code and subsequently executes a RST 0. Any program can be aborted safely using CTRL-ALT-ESC. Note that the PEEK value of address 23 is preserved (or at least, it should be!).

On some computers, CTRL-ALT-ESC is already in use for system purposes; therefore, the keypress CTRL-SHIFT-ESC may be used as an alternative.

In general, CTRL-ESC should only be used when the normal Break is disabled. If you use CTRL-ESC in the normal ZX81 editor, it will lock keyboard operation. Press CTRL-ESC once more to get proper key response again. In the CBI editor, CTRL-ESC merely produces a space character, i.e. does not lock the keyboard. In all cases, avoid using CTRL-ESC when CBI Trace is active.

CTRL together with a key other than one of the above has the effect of SHIFT pressed together with that key. CTRL-C is hooked and has no drastic effect (it produces Shift-C, i.e. a question mark). CTRL-Break is more dangerous: it immediately stops XTender and returns to the DOS prompt.
 

KEYBOARD COMPATIBILITY

For unShifted PC-keys, XTender maps almost the entire PC keyboard to the ZX81 keyboard. Not only the 40 basic ZX81 keys, but also things like the arrow keys, backspace and the numeric pad are mapped. For example, if you press 'arrow left', XTender will convert this to Shift-5, the ZX81 equivalent. This makes editing (and sometimes also playing games) less strenuous. The central key of the numeric pad ('5') generates Shift-Enter, i.e. toggles Function mode (a special cursor mode for obtaining functions tokens, see Keyboard Facilities). More special keys: the square brackets [] are converted to regular brackets () and the single forward quote ' is converted to the ZX81 double quote ". The ESC key generates Shift-1 (Edit), i.e. brings the current cursored Basic line down into the ZX81 line-editor.

Also for Shifted keys, the emulator helps you a little bit. Pressing any one of the Shift keys will always give you the ZX81-Shifted symbol (like Shift-U for '$'). But, if you press Alt instead of Shift, you will get the PC-Shifted symbol, so you can press Alt-4 to get the Dollar $ign. Some non-trivial ALTernatives are listed below:

• ALT-6 gives ** instead of ^
• ALT-7 gives AND instead of &
• ALT-[ gives <= instead of {
• ALT-] gives >= instead of }
• ALT-' gives "" instead of "

Finally, if CAPSLOCK is on, the effect of Shift is inverted for all keys (not only the letter keys). So, pressing B with CAPSLOCK on gives *, and Shift-B gives B. Note that this can be especially nice for some games which use the keys 5 through 8 for steering and do not allow you to press Shift (so you cannot use the PC arrow keys). With CAPSLOCK on you can use the arrow keys of the PC-keyboard!

Multiple keypresses are handled correctly, which is especially useful when playing games (e.g. move and fire at the same time).
 

MOUSE CONTROL

If you wish, you can also use your PC mouse to control a few of the ZX81 keys. This is an experimental feature and therefore it needs to be enabled at start-up by the /x commandline qualifier.

When enabled, mouse movement and clicks can be used to "press" ZX81 keys, as shown in the table below:

This works well for the ZX81 BASIC program editor and for many ZX81 games. In a future version of XTender, the ZX81 keys may become programmable. If you have suggestions on how to improve this feature, please let me know.
 

FILE MANAGEMENT

To SAVE or LOAD a program, just type

• SAVE "name"
• LOAD "name"

LOADing a non-existing program will display the proper DOS error. If you SAVE a program which already existed, it will simply be overwritten without any user confirmation. Saving a program without a name (empty quotes) results in the usual error F. Originally, QZ filenames were up to 12 characters long. In DOS, one is so unfortunate to be restricted to 8. So: only the first 8 characters of the name will be used. Do not include inverse characters in the name, since these are used internally by the ZX81 to indicate the end of the name.

Most ZX81 users will be used to typing LOAD "" for loading the first program found on tape. Since diskdrives are not designed for sequential access, LOAD "" cannot be used on the emulator. Instead, XTender will show the directory of the current drive and path (same as *DIR:, see below).

QZ file handling commands can be passed to XTender in a LOAD or SAVE statement. The first character of the SAVE/LOAD name must be an asterisk * to indicate that one or more QZ commands follow (instead of the usual filename). The available commands, a short description and examples of each command are listed below:

• BACK:
• Go one level back in current directory path; in fact equal to CD:..: but included for compatibility reasons.
• Example
• LOAD "*BACK:"
• *CD:
• Change current directory to indicated path name; note that the path name ends with a colon : (as in all other QZ commands), so you cannot use CD: to change the directory on a non-default drive; apply the command USE: first. The usual DOS backslashes must be replaced by forward slashes.
• Examples
• LOAD "*CD:GAMES:"
• LOAD "*CD:/ZX81/ASM/TOOLS:"
• LOAD "*CD:..:" LOAD "*CD:/:"
• *DEL:
• Delete indicated file; the extension of the filename (i.e. the filetype) must be typed first, followed by a colon : and the main part of the name; the command is ended by a colon as well. The filetypes recognized by XTender are B, C, L, P, S and V. Note that only files in the current drive and path can be deleted. For deleting files on other drives or paths, use CD: and USE: first. DEL: is the same as REMOVE:
• Example
• LOAD "*DEL:P:RUBBISH:" (deletes RUBBISH.P)
• *DIR:
• Show the ZX81 files on the current drive; all the specific ZX81 files (with extensions .B, .C, .L, .P, .S or .V) are listed, together with their length, date and time; note that the date is shown in the format "yy-mm-dd", not in the more usual DOS format "mm-dd-yy"; the time is shown in 24-hour format. Subdirectories without extension are shown too. At the end of each page, press any key to see the next page or Q to stop the directory listing; at the top of each page you will see the current drive and path name; the backslashes are replaced by forward slashes; at the end of the file list you will see the total number of files and the number of clusters left on the current device (depending on your drive's FAT, a cluster can hold 512 bytes, 1k, 2k, 4k, etc.).
• Examples
• LOAD "*DIR:"
• LOAD "" (convenient alternative!)
• *KILL-XTR:
• Stop the current XTender session, return to DOS.
• Example
• LOAD "*KILL-XTR:"
• *LOAD:
• Load one of the special filetypes (not P-files!). The filetypes available are:
• B for basic
• all the lines of the current Basic program are deleted and the the file is then inserted in the Basic area. The variables area remains unchanged. XTender does not check if there is sufficient memory available to insert the file. If not, a crash will result. If all is OK, error 'I' will be generated (data merged).
• Example
• LOAD "*LOAD:B:SORTING:"
• C for code
• the code file is loaded from file to the address specified; XTender tries to load all the bytes from the file but stops after exceeding address 65535.
• Example
• LOAD "*LOAD:C:CHARSET:8192:"
• L for lines
• the lines in the file are merged with the current Basic program. If a line with a certain number already exists in the current program, it will be replaced by the new line with the same number. The variables area remains unchanged. XTender does not check if there is sufficient memory available to insert the lines. If not, a crash will result. If all is OK, error 'I' will be generated (data merged).
• Example
• LOAD "*LOAD:L:PARTLY:"
• S for screen
• the screen file is loaded from file into the current ZX81 display file. It is assumed that the display file is fully expanded; otherwise: ZX81 crash!
• Example
• LOAD "*LOAD:S:PICTURE:"
• V for variables
• the variables area of the ZX81 is cleared, and then the file is inserted in this area. The Basic program remains unchanged. XTender does not check if there is sufficient memory available to insert the file. If not, a crash will result. If all is OK, error 'I' will be generated (data merged).
• Example
• LOAD "*LOAD:V:GRAPHS:"
• *MAIN:
• Go to root directory of current drive; in fact equal to CD:/: but included for compatibility reasons.
• Example
• LOAD "*MAIN:"
• *MD:
• Make new directory as indicated by path name; note that the path name ends with a colon : (as in all other QZ commands), so you cannot use MD: to create a directory on a non-default drive; apply the command USE: first. The usual DOS backslashes must be replaced by forward slashes.
• Examples
• LOAD "*MD:JUNK:"
• LOAD "*MD:/ZX81/GAMES/SHOOT:"
• LOAD "*MD:../LEVEL2:"
• *RD:
• Remove directory as indicated by path name; note that the path name ends with ':' (as in all other QZ commands), so you cannot use RD: to remove a directory on a non-default drive; apply the command USE: first. The usual MS/DOS backslashes must be replaced by forward slashes.
• Examples
• LOAD "*RD:OLD-DATA:"
• LOAD "*RD:/ZX81/TAPE:"
• LOAD "*RD:../BACKUP:"
• *REMOVE:
• Remove indicated file; the extension of the filename (i.e. the filetype) must be typed first, followed by a colon : and the main part of the name; the command is ended by a colon as well. The filetypes recognized by XTender are B, C, L, P, S and V. Note that only files in the current drive and path can be deleted. For deleting files on other drives or paths, use CD: and USE: first. REMOVE: is the same as DEL:.
• Example
• LOAD "*REMOVE:C:ZXASSEM:" (removes ZXASSEM.C)
• *ROOT:
• Go to root directory of current drive; in fact equal to CD:/: but included for compatibility reasons.
• Example
• LOAD "*ROOT:"
• *SAVE:
• Creates one of the special-type files (not P-files!). The filetypes available are:
• B for basic
• all the lines of the current Basic programs are written to file. System variables, Basic variables and screen are not written to file; a B-file is a compact way for storing short programs on disk.
• Example
• LOAD "*SAVE:B:QUICK:"
• C for code
• the code block starting at the address determined by the first parameter and with a length determined by the second parameter is written to file. The length must be larger than 0 and the calculated end address may not exceed address 65535. Please note that the second parameter does NOT represent the end address.
• Example
• LOAD "*SAVE:C:CHARSET:7680:512:"

(saves the contents of addresses 7680 through 8191)
• L for lines
• those lines of the current basic program with a line number lying within the interval determined by the two parameters supplied are written to file. The first line number must be smaller than or equal to the second line number. The second line number must be smaller than 10,000. The two line numbers need not actually exist in the current program. If no lines lie in the specified interval, a file of zero length is produced.
• Example
• LOAD "*SAVE:L:ELEPHANT:1000:2000:"

(saves all lines with numbers greater than or equal to 1000 and less than or equal to 2000)
• S for screen
• the current ZX81 display file is written to file. It is assumed that the display file is fully expanded; this means that the resulting S-file is always 793 bytes in length (namely 24*33+1).
• Example
• LOAD "*SAVE:S:DISPLAY:"
• V for variables
• the current contents of the ZX81 variables area are written to file (including the delimiting CHR$(128)). Directly after a CLEAR (or similar) command, this area is empty and the file will have length 1.
• Example
• LOAD "*SAVE:V:TAXES:"
• *SHOW:
• Shows information about the current version of XTender plus a list of all QZ commands available.
• Example
• LOAD "*SHOW:"
• *USE:
• Selects another drive as default drive. If you have only one physical floppy drive, which can be accessed logically as A: or B:, XTender will always use the last logical setting of this drive to avoid the DOS message requesting to insert a disk (very few other programs I know also do this!). But note that this only works from DOS 3.2 onwards. A path name cannot be included, use CD: instead.
• Example
• LOAD "*USE:D:" (selects drive D)

Only the trailing colon of a line may be omitted (i.e. the one behind DIR in the example above). Any non-supported command after the asterisk will display the erroneous text plus the message ERROR IN QZ COMMAND, and stops with Error C (syntax error).
 

PRINTER OUTPUT

HIGH RESOLUTION

Hi-res is detected by XTender by trapping an immediate change in value of register IX (used by the ZX81 as NMI server address). The term 'immediate' means that only the instruction LD IX,nn (DDh.21h) is trapped. You could avoid the trap by using a sequence like LD HL,nn, PUSH HL, POP IX. However, all hi-res programmes tested use the (more obvious) immediate method.

After the detection, XTender tries to emulate the semi hi-res by looking for the characteristically shaped 6k display file in the bottom 16k of RAM and printing it to the PC screen appropriately. If it cannot locate this display file, normal resolution will be switched on.
 

ZX81 64K MEMORY MAP

Note: [addr] is the value obtained by PEEK(addr)+256*PEEK(addr+1), i.e. the usual Z80 word-peek with the low byte logically in front of the high byte.
 

ROM AND 8-16K AREA

At startup, the 8-16k area contains a relocated version of the Coral Basic Interpreter (CBI version 7.0). You can start this program by typing

This should give 0/0 in line 22 and a black cursor in line 23. CBI may be useful for people with little ZX81 experience since it has both a full-entry keyword decoder, full-screen editor and explains error messages on request. All usual ZX81 commands are available, as well as 46 new ones (including commands for PROCedures with two-way data exchange and structured loops). Naturally, CBI was written for the ZX81, but runs equally well on XTender. For details on this program, read the textfile supplied with XTender. Two useful commands to enter when the black cursor appears are

Note that you have to type the letters one-by-one, the spaces are optional. The system variables of CBI are stored at address 65400 onwards (corresponding to address 32730 in the original version). The CBI program ends at address 13522, so the part of memory from address 13523 up to and including 16383 is available for your own use.
 

STOPPING XTENDER

which will bring you back to DOS. A fast alternative is:

with STOP typed as a keyword (i.e. Shift-A) instead of the separate letters.
Finally, for the very impatient: pressing CTRL-Break will stop XTender as well!
 

CLOCK FREQUENCY

The ZX81 runs at approximately 3.2 MHz in FAST mode and (effectively) at 0.8 MHz in SLOW mode (emulated by XTender). Supposing a program on XTender takes half the time the ZX81 would take to execute it in SLOW, the effective clock frequency is (0.8 MHz)/(0.5) = 1.6 MHz.

Another way to find the clock frequency is to use a machinecode test loop, embracing a single instruction which is executed many (e.g. 65536) times. If you then count the number of frames needed (and subtract the number of frames needed for just the empty loop) you can get an idea of the clock frequency of that specific instruction, provided you know the number of T-cycles the Z80 would normally need for that same instruction.

A very good estimate of the emulated processor speed can be obtained by LOADing the program CLCKFREQ. This program does some simple printing and plotting, but the final result is a reliable measure for the average speed of XTender and the effective (emulated) processor speed.
 

SLOWING DOWN

On fast machines the speed of XTender can be reduced by POKEing address 23. The byte at this address is used as delay counter at each Z80 instruction. Default value is 0, which gives (practically) no delay. A higher value will quickly slow-down the emulation process.

Values 1..128 yield a 'linear' slowdown: after each Z80 instruction, the emulator will be halted some time; the length of the idle period is linearly dependent on PEEK 23. When POKEing 128 into address 23, XTender will be running about 27 times as slow as when you would have POKEd value 0.

POKEing values 129..255 results in a 'quadratic' slowdown and should be used on very fast machines only; even values only slightly above 128 can result in a huge slowdown factor. Theoretically, the maximum slowdown factor is about 3500 which is obtained by POKEing 255 into address 23 (but unfortunately I didn't have enough time for spare to test it...)

Please note again that the slowing down does not work with XTENDER! (with exclamation mark, supplied in the shareware package), only XTENDER (without exclamation mark, supplied in the registered package) can be slowed-down.

Once you have determined which value of PEEK 23 suits your needs best, you can start XTender with the /d:nnn qualifier in order to set the delay factor to nnn without actually having to poke it into the ZX81 memory (see Commandline Qualifiers). Of course, you can still alter this initial value during the session!
 

Z80 EMULATION

Some brief remarks for Z80 freaks:

• Register I points to the ZX81 character set. The default value is I=1Eh which means that the character set starts at address 1E00h (last half kilobyte of ROM). If you want to change the character set, first POKE the new set somewhere in memory (at an address which is a multiple of 512 bytes) and then alter register I accordingly (so: only even values for I are allowed). This will instantaneously change the display. The reversed sequence of this procedure will not work the way you would expect! I advise not to change the character set present in ROM.
• The Subtract flag (N) of the Z80 flag register (f) is not emulated. Many Z80 instructions alter this flag, but there is only one instruction which uses the result, viz. DAA. Since DAA is used only once in the ZX81 ROM (where it has been replaced by an equivalent non-standard Z80 instruction) and since proper emulation of the Subtract flag reduces operation speed of XTender, I think this exclusion is a sensible decision. When XTender encounters a DAA instruction, it always performs a 'Decimal Adjust After Addition'. So, keep your fingers crossed!

Some programs using DAA are: Reversi (only the machine coded version), MCTT (Machine Code Testing Tool) and Frogger. If you should know of other programs which use DAA, please tell me!
• The IN instructions are all neglected, except for requests for port FEh (the keyboard) and port FBh (the printer). Also, OUT instructions are neglected. Note, however, that the instructions IND, INDR, INI, INIR, OUTD, OTDR, OUTI and OTIR alter the registers BC and HL (and also (HL)'s) appropriately!
• HALT waits for an interrupt. Interrupts are not emulated and therefore HALT is made equal to NOP.
• Some instructions cannot be used for changing the ROM area (0-8k). These are: LD (HL),A, LD (DE),A, LD (HL),D, LD (HL),E and LDIR. The reason for this is to protect the ROM against itself! The programmers of the ZX81 ROM have made use of the write-protection of the 0-8k area for simplifying their code (or is it a hitherto unknown bug of the ZX81 ROM?). So, XTender (which has no write-protection in the ROM area) would alter its own code, which is (in general!) a bad thing to do. Therefore, the above instructions are emulated such that they have no effect in the first 8k of memory. Note that this also protects the ROM against a program like MCODER-2, which does the same kind of nasty things to the ZX81 ROM-area...

PROGRAMS FROM TAPE TO DISK

SCREEN-SAVER

On registered copies, the screen-saver can be disabled by using the /s qualifier at startup (see section Commandline Qualifiers for more information).
 

WARRANTY

ABOUT THE AUTHOR

In 1987, I decided to buy a Sinclair QL (during sales) and found this younger brother to be just as interesting as the ZX81. It also gave me a good excuse for learning 68000 machine code. As I saw that less and less people were actually using the ZX81, I sought a way for elongating its existence. I thought that (among other things) a ZX81 emulator for the QL would be a good way to do so, and I started writing this program (called XTricator) in the end of 1989.

At that point in time, it also became unacceptable to not have a PC - so I bought one. In order to stay loyal to the absorbing machines made by Sinclair, I decided to write a ZX81 emulator for MS/DOS as well. The 8086-family of processors is (basically) very similar to the Z80, so I did not really have to learn a new language, only a new OS. The first beta version of XTender was already available in 1991, and the first official release was in 1993.

By now, in 2006, you can see the final and ultimate result of my efforts. Although XTender is still a "simple" DOS program in a world ruled by "Windows" and "clicking", I sincerely hope that many (former) ZX81 users will take some time to use XTender to leap back in history and (re)enjoy the overwhelming simplicity of the 64k, 8-bit, 3.2 MHz Z80, black & white computer with 64x44 graphics resolution, which introduced over a million people world-wide (including myself) to the wonderful world of computing!
 

XTENDER RELEASE HISTORY & FUTURE

FINAL NOTES