The G007
Hi-Res Graphics pack
for the ZX81

User Manual

Circuit Design: Gary Keall

Contents

Conditions Of Use

The G007 requires the following:

1. An external RAM pack, of 8K or more.

2. The 1K or 2K internal RAM must be present.

3. Any additional hardware used with the module must reside between the G007 module and the RAM pack.
The exception is the ZX81 printer, which fits between the G007 and the ZX81.

4. The memory between 2000H and 3000H may only be used by the G007.
This does not stop 64K RAM packs being used, but the G007 will disable this area of memory in the RAM pack. Due to the many add-ons available, we cannot guarantee the G007 to work with any add-on other than the Sinclair RAM pack and printer. However, most of the alternative RAM packs, including 64K ones, should be fine.

1. Introduction

Welcome to the world of high-resolution graphics! The G007 provides a 256x192 pixel display mode and graphics command extensions to the BASIC interpreter.
Over 100 different operations may now be performed with the modified statements:

PLOT n,x,y , CLS n, COPY n, SLOW n, FAST n

These are all multipurpose statements, now, with the different functions being selected by the new parameter 'n'. The really versatile statement is PLOT n,x,y which can now do such things as line drawing and textured area filling. The G007 also provides powerful facilities for printing on the high-res screen, using the PRINT statement.

User defined characters can be used, allowing space invaders, lower-case etc., and text may be placed at any high-res screen position.

This manual contains a brief introductory section, describing the main features of the high-res display mode, and gives a few notes to get you started with the G007 commands. Most of the rest of the manual forms a reference guide to the facilities offered by the G007 module. The appendices at the end of the manual summarise the BASIC statements, and describe how the memory used by the G007 is organised.

Finally, a few sample programs are included, to demonstrate the capabilities of the G007 High-res graphics.

1.1.The High Resolution Display

The new high-resolution display mode of the G007 graphics module is in addition to the existing low-res mode. The ZX81 may switch freely between them without losing the contents of either. The high-res display requires 6.4 K of RAM, and is automatically set-up by the first hi-res command after applying power.

In hi-res mode, the screen has two parts, just as in lo-res. The top part is the actual hi-res display, and is always the full 256x192 pixels. The lower part is quite different, just a single text line which is actually the bottom of the lo-res screen. This line is displayed only while a program is not running, and is used to show reports, input data or commands entered at the keyboard.

1.2.Setting Up

While not powered, plug the G007 between the ZX81 and its external RAM pack. The latter is essential. The ZX printer may be used, and plugged between the ZX81 and the G007.

After applying power, all G007 commands can be used immediately, since all hi-res details are set-up automatically. If there is not enough memory, error 4 results.

Existing programs which do not use PLOT will LOAD and RUN without modification

1.3. Getting Started

Read the following to avoid confusion. All G007 commands can be used in programs or as commands, but the rest of this section assumes command mode.

To enter hi-res mode, simply enter SLOW 2 and note the ZX81 will now accept a number after SLOW. The report: 0/G007 shows the display has been set up. The following command sequence should give you an idea of what is now possible:

CLS 1
CLS 2
PLOT 2, 150, 60
PLOT 2, 30, 180
PLOT 74,60,60
PLOT 42,0,0
CLS 3

N.B. If the top of the screen bends to the right, you may correct this with POKE 8833,0.
You may have gathered that PLOT 2 draws lines, while PLOT 42 and PLOT 74 fill triangles. These commands, and many variations, are explained in the rest of the manual.

The high-res screen is not cleared between commands. It is not even cleared by RUN, so a CLS 1 or CLS 2 statement is usually needed at the start of every hi-res program. Programs cannot be listed on the hi-res screen, so they should be entered in lo-res mode. Entering programs in hi-res mode may have some odd but harmless effects.

SLOW returns to lo-res mode.

LIST will end with G007, which is actually the "number" of a program line containing the hi-res display file. This is ignored by the BASIC interpreter. This line cannot be listed, deleted or edited.

You may wish to try some of the example programs in the appendix.

2. The BASIC Statements

Most of these work by adding an optional extra parameter n to the existing BASIC graphic commands. Parameters can be any numeric expression, but they will be rounded to the nearest integer. All of these commands (except PLOT) will work in the original way if n is omitted.

PRINT only prints the hi-res display if hi-res mode is set. Other commands imply which display file is used, and hence it does not matter which mode is set.

2.1. Setting High or Low Resolution Mode

These two statement always set the relevant SLOW or FAST mode, and select the display mode. With n=0, or n omitted, low-resolution mode is selected. With n=1 to 6, high-res mode is selected, and n also determines the mode of operation for the PRINT statement.
The most commonly used hi-res mode is SLOW 2 (or FAST 2), in which PRINT will operate in the normal manner, albeit on the high-res screen. The other more advanced PRINT modes are explained later.

2.2. Clearing the Display

This is the only statement that can clear the high-res display, and should always be done at the start of a program. CLS N will also set the graphics origin to the bottom left of the screen.

CLS 0 clears the low res display
CLS 1 clears the high res display to black
CLS 2 clears the high res display to white
CLS 3 inverts the high res display
CLS 1 and CLS 2 also reset the PLOT position to 0,0 at the bottom left corner of the screen (see section 4 for an explanation of these terms).

2.3. Copying the Display file to the Printer

COPY copies the lo-res display to the ZX printer.
COPY 0 copies the lo-res display to the ZX printer.
COPY 1 copies the hi-res display to the ZX printer.

If the printer is not fitted, this command is ignored.

2.4. Point Plotting, Line Drawing and Triangle filling

PLOT n,x,y is the real work-horse of the G007, and behaves quite differently to the old PLOT. The parameter n can range from -1 to 130, giving 115 separate functions for the statement. The large number of options arises from having several modes for plotting individual pixels in lines, prints and triangles, and form the alternative means of specifying co-ordinates. The statement also incorporates such advanced features as different line types for drawing lines and for textured triangle filling.

The Old PLOT and UNPLOT Functions

The original functions of PLOT and UNPLOT have been retained, but only for use on the low-res display. PLOT x,y has been replaced by PLOT -1,x,y and UNPLOT x,y has been replaced by PLOT 0,x,y. The UNPLOT statement now does exactly the same as PLOT.

Co-ordinates

The co-ordinates x and y in the PLOT statement specify a position on the screen called the 'PLOT position'. More than one PLOT position is needed to draw a line or triangle, so the ZX81 remembers the PLOT positions from the previous two PLOT statements.

The PLOT statement can use either absolute or relative co-ordinates.
Absolute co-ordinates are x pixels to the right and y pixels up from the current graphics origin. Relative co-ordinates are x pixels to the right and y pixels up from the previous PLOT position. The hi-res screen is 256 pixels wide by 192 pixels high, and the graphics origin is normally located at the bottom left of the screen. Hence x has a range from 0 to 255, and y ranges from 0 to 191. The PLOT position may lie off the screen (except when filling triangles) and the co-ordinates in the PLOT statement may take any value, even negative ones. Note that it is very easy to run off the screen using relative co-ordinates.

Plotting Points

Individual pixels may be plotted in 3 basic modes: white, black, or inverted (also known as exclusive-or). These modes are also available in line drawing and triangle filling. PLOT may also be used to simply move the PLOT position without affecting any pixels. The effect of PLOT n,x,y for the following values of n are as follows:

Drawing Lines

Lines may be drawn in white, black or invert modes. the line is drawn from the previous plot position to the current plot position. The line is drawn even if one of the ends lies off the screen.

Using invert mode, the line may be drawn missing the last point, so that successive lines may chain together properly. Three other types of line are available, giving dotted and dashed lines. These are useful for identifying different lines when drawing graphs.

Note that broken line types are obtained by adding 32, 64 or 96 to the values of n for solid line types.

Filling Triangles

Triangle filling statements are similar to line drawing ones. The vertices of the triangle consist of the current and the previous two PLOT positions. Triangles are filled only if they lie completely in the screen area. This is to achieve a reasonable speed by using 8-bit arithmetic (instead of the usual 16-bits). If any vertices lie off-screen, an error B results.

Using invert mode, triangles may omit the last edge (between the current and previous PLOT positions) so that adjacent triangle can join up properly. There is also a textured triangle fill which makes shading, cross-hatching, etc. possible. The texture is user-definable, making this a very powerful facility.

The default texture is a chequer-board pattern, giving a grey effect. The texture can be re-defined by poking the bytes at 8970 and 8971. The effect of these bytes is described in the following section on advanced PLOT facilities, but poking 8970 with 1 or 17 gives a coarse or fine diagonal shading.

2.5 Advanced Plot Facilities

Moving the Graphics Origin

As mentioned earlier, the graphics origin is normally located at the bottom left corner of the screen. It can be repositioned by the statement PLOT 130, x, y. The co-ordinates x and y give the new position of the graphics origin relative to the bottom left corner of the screen.

Filling In Shapes And Drawing Radial Lines

The PLOT statement has a special facility to make it easy to fill circles and other shapes using triangles. This allows pie charts etc. to be easily produced, and can also be used for drawing radial lines.

Normally, the current PLOT position of a statement will be move to the 'previous' PLOT position when it has completed. At the same time, the 'previous' plot position is moved to the 'twice previous' PLOT position. In effect, the PLOT position is moved into a queue and discarded after two more PLOT statements.

If 16 is added to the parameter n, the queue-shifting sequence is different. On completion of the current PLOT statement, it is saved as the 'twice previous' position, instead of the 'previous' position. This has the effect the the 'second' vertex of successive triangles will remain at the same point, or the starting point of successive lines will be fixed. This makes it simple to fill in shapes, such as circles, with triangles. The PLOT position is moved to a point within the shape (using PLOT 12,x,y) and then triangles are plotted by moving around the periphery of the shape (using PLOT 58,x,y for instance). Example program 1 demonstrates this facility, along with other forms of the PLOT statement.

Triangle Texturing

Triangle textures are defined by bytes 8970 and 8971. These are rotated by 1 and 3 bits to the right, respectively, for each new row of the triangle. They are then ORed together, and the resulting pattern is used to fill that row of the triangle.

More complex patterns can be obtained by POKEing these bytes. You can experiment to find effects.

Further Line Types

The four standard line types are determined by the bytes 9012 to 9015 respectively. These can be redefined by POKEing, the line pattern being the inverse of the bit pattern.

Read Point Function

Any pixel may be read by moving the PLOT position to that pixel (using PLOT12,x,y) and then PEEKing 8983. If the number is zero, then the pixel is white, else a single bit of this byte will be set indicating the pixel is black.

Printing on the High Resolution Screen

The high resolution screen allows great scope for printing character. They may be placed anywhere on the screen, no longer limited to the normal 32x24 character grid. Also, new character shapes can be defined, and there is no liit to the number of characters that may be used in the same display. You can even superimpose graphics and characters.

The G007 provides six different PRINT modes when using the high-res screen. The PRINT mode set is given by the parameter n that was used in the statement SLOW n or FAST n that set the high res mode.

In all PRINT modes, text can be printed right down to the bottom of the high-res screen.

Note that the PRINT statement will only print string results. If you need to print a numeric result, you must first convert it to a string by preceding it with STR$.

The SCROLL statement is completely inoperative in high-res mode.

PRINT modes 1 and 2 work just as in low res mode, except that in mode 1 the character pixels are reversed (white foreground, black background).

PRINT modes 3 to 6 print at the position of the last PLOT statement. The PLOT position is moved after each character is printed. The first character will be printed with its top left corner at the PLOT position. If the PRINT statement ends in a semicolon, the PLOT position will be moved one pixel to the right of the top right corner of the last character printed, else it will go to the pixel below the bottom left corner of the first character printed. This ensures character printing places text in a similar manner as the low-res screen.

To use these modes, the PRINT statement is usually preceded by a statement to set the PLOT position (e.g. PLOT 12,x,y or PLOT 16,x,y). The PRINT AT and TAB work only in low-res, not high res mode.

Note that characters may be superimposed, as only the foreground (or background in mode 5) is printed.

User Defined Characters

Because the high-res PRINT modes allow characters to be printed in normal and inverse video, the old ZX81 inverse video characters (CHR$ 128 to 191) are redundant and thus free for redefinition.

The G007 allows the 32 characters from CHR$ 160 to 191 to be readily redfined, and reserves memory for this purpose. You can actually redefine all 128 characters, but you must allocate memory to do this - see Appendix B.

The character table containing the definitions of the 32 new characters is stored in 256 bytes of RAM starting at address 8448. This area is fully protected from NEW et.c Each character definiton consists of eight bytes, corresponding to eight rows of pixels in the character. The eight pixels in each row of the character correspond to the 8 bits (binary digits) in each byte, with the most significant bit at the left.

To redefine a character, you need to work out the eight bytes. This is best done by drawing the character first. The numbers should then be poked into eight consecutive memory location in the appropriate place in the character table. For character number K, the first location is at 8448+8*(160-K), where K is between 160 and 191 inclusive. Locations outside 8448 to 8703 must not be POKEd, or the computer may crash.

Example program 2 shows in detail how the step involved in defining a new character.

The redefined characters can only be printed in high-res mode of course. In low-res mode, the original character will be obtained. It is usually best to print the new characters using CHR$ K, to avoid confusion.

The inverse characters 'A' to 'Z' could be redefined as lower case. If so, then if the inverse video characters are used in PRINT statements when the program is typed in, these will appear in lower case when they are printed to the high-res screen.

If a user definable character is printed before it has been defined, a 'garbage' character will appear.

2.6. Miscellaneous

Saving High Resolution Pictures

The high-res display file is normally deleted automatically by the SAVE command, so that excesive time is not spent saving the 6K of display data. The high-res display (and program) can be saved by directly calling the saving routine, with RAND USR 764. FAST mode must be set to do this. The program will be saved without a name, so will have to be loaded using an empty string as a name, i.e. with LOAD "". Before loading the program, make sure a high-res display file is already set up (e.g. by typing CLS 2), otherwise the newly loaded display will be deleted by the first high-res statement.

Deleting The High Res Display File

This can be done by USR 11737, at any time. This could be done to aquire extra memory for example.

Correcting Television Picture Distortion

If you find the top of the TV picture bends slightly to the right,
you may be able to correct this with POKE 8833, 0.