PROGRAM.. PATCH.BAS – LOAD FROM BASIC
THIS DOCUMENTATION….. PRINTED FROM PATCH.PD
The version of BBC BASIC resident in the cambridge computer z-88 is deficient in a few respects, especially in the lack of any editing facilities or support for graphics operations. The purpose of the BASIC Patch program is to provide some of these capabilities.
The PATCH program is installed with a simple CHAIN command, and thereafter remains resident until the machine is reset or BASIC is KILLed. It occupies 2 kb of memory, and results in the value of PAGE being raised to &2B00; when the graphics operations are used a further 2k is used for the display buffer.
The PATCH will work only on an expanded machine, (at least 128kbytes of RAM installed in slot 1). If this is not the case, or if the available memory has been deliberately reduced by changing HIMEM, the message “NO RAM” will be displayed when the program is CHAINed.
The new features provided by the PATCH.BAS program are as follows:
*EDIT line number
This command allows you to edit a specified program line. It results in the line being displayed, (after a short delay), with the cursor positioned at the end of the line, and you can edit the line using any of the usual line-editing features, as follows:
left Move cursor left one character right Move cursor right one character SHIFT left Move cursor left one word SHIFT right Move cursor right one word <> left Move cursor to start of line <> right Move cursor to end of line DEL Backspace and delete SHIFT DEL Delete character under cursor <> DEL Delete entire line <> D Delete from cursor to end of line <> G Delete character under cursor <> S Swap case <> T Delete up to next space <> U Iinsert space at cursor position <> V Toggle between insert and overtype
To enter the edited line into the program press ENTER; to abandon the edit and leave the line unchanged, press ESC
You can also use *EDIT to concatenate, two or more sequential program lines, by specifying the first line separated by commas,(eg.*EDIT 10,30) In this case, you will have to edit out the line nos. of the second and subsequent lines; then delete the old line nos.
*EDIT may be abbreviated as *E. (the dot is required)
MODE n
The MODE statement allows selection of the normal text-only mode (MODE 0), or a text-and-graphics mode (MODE 1). In MODE 1, the display is split into two parts: a text-window on the left and a graphics-window on the right. The text window consists of 8 rows of 50 characters, and the graphics window is 64 pixels high by 256 pixels wide; you cannot (normally), mix text and graphics in the same window.
Points in the graphics window are addressed as x,y coordinates from 0,0 (the bottom-left corner) to 255,63 (the top-right corner). The origin can be moved from 0,0 using the PLOT-1 statement (q.v.)
Although MODE 1 sets up the window positions and sizes as described, it is possible to change these using the VDU statement. However, the method of doing this is outside the scope of this document. It is not advisable to cause the text and graphics windows to overlap, although this may occasionally be useful.
MODE clears the display (both text and graphics windows), moves the text cursor to 0,0 (the top left of the text window), resets the graphics origin and moves the graphics cursor to 0,0 (the bottom left of the graphics window).
In MODE 0, (the normal 94-column text mode), the other graphics statements have no effect.
CLG
This clears the graphics window only; it does not affect the position of the graphics cursor. Note that CLS can be used to clear the text window and leave the graphics window unchanged.
DRAW x,y
Draws a straight line (in black), between the current position of the graphics cursor and the specified coordinates, then moves the graphics cursor to the specified position.
This statement is identical to PLOT 5.
MOVE x,y
Moves the graphics cursor to the specified coordinates, but does not affect what is displayed.
This statement is identical to PLOT 4.
PLOT n,x,y
A multi-purpose plotting statement, whose effect is controlled by the first parameter n :
n action
-1 Move the graphics origin to x,y
0 Move the graphics cursor relative to the last point
1 Draw a line, in "black", relative to the last point
2 Draw a line, in "inverse", relative to the last point
3 Draw a line, in "white", relative to the last point
4 Move the graphics cursor to the absolute position x,y
5 Draw a line, in "black", to the absolute position x,y
6 Draw a line, in "inverse", to the absolute position x,y
7 Draw a line, in "white", to the absolute position x,y
8-15 As 0-7, but plot the last point on the line twice
(i.e. in the "inverting" modes omit the first point)
16-31 As 0-15, but draws the line dotted
32-63 As 0-31, but plot the first point on the line twice
(i.e. in the "inverting" modes omit the last point)
64-71 As 0-7, but plots a single point at x,y
72-79 Draw a horizontal line left and right from the point
x,y until the first "lit" pixel is encountered, or
the edge of the window. This can be used to fill
shapes.
80-87 Plot and fill a triangle defined by the 2 previously
visited points and the point x,y.
88-95 Draws a horizontal line to the right of the point x,y
until the first "unlit" pixel is encountered, or the
edge of the window. This can be used to "undraw".
96-103 Plot and fill a rectangle whose opposite corners are
defined by the last visited point and the point x,y.
POINT (x,y)
This function returns the state of the pixel at the specified location, as 0 (unlit) or 1 (lit). if the specified point is outside the graphics window (taking into account the position of the origin), or if MODE 0 is selected, the value -1 is returned.
The graphics statements COLOUR and GCOL are not implemented.
Installation of the BASIC patch has a number of “side effects” that you should be aware of…..
1. Changing HIMEM will have the effect of disabling the patch. If HIMEM has been changed it must be set back to &C000 before re-CHAINing the patch program (or alternatively KILL and re-start BASIC from the INDEX). Changing HIMEM is not recommended in any case, since setting it to an unsuitable value will “crash” the machine (even without the patch).
2. The “Silly”, “RENUMBER space” and “LINE space” errors will not appear; instead, the “No room” message will be produced in each case.
3. You are advised to select MODE 0, before entering PIPEDREAM, since it seems to get confused by the presence of the graphics window.
4. Since Cambridge Computer provided no “legal” method of installing a patch such as this, (indeed, for a long time I thought it would be impossible!!), a rather “dirty” method has had to be adopted. This has some unfortunate, but unavoidable, consequences:
(A) The RUN and CHAIN commands, may occasionlly fail to work properly (on average less than 1 in 1000 times). If this happens no harm will be done; simply issue the command again.
(B) If you reply to the INPUT statement with a very long string. (more than 252 characters) the machine will crash, so you must avoid doing that.
5. Using graphics statements in an ON ERROR routine may give anomalous results. example:
10 ON ERROR MODE 0 : REPORT : END
20 MODE 1
30 REPEAT
40 DRAW RND(256)-1,RND(64)-1
50 UNTIL FALSEThe above program can be exited only by peressing ESCape. The intention is that this will cause the display to clear and the message “Escape” to be displayed. In practice, the message actually displayed will be “Sorry, not implemented” since, although the patch is active, the MODE statement still affects REPORT, ERR and ERL.
The patch has been tested only with EPROM Operating System version 2.2 although it should also work with ROM version 3.0. Since it of necessity depends on certain specific features of the Operating System remaining unchanged, I cannot guarantee that it will operate correctly (or at all) with other versions.
The version of PATCH.BAS includes the powerful printing command called screen dump code, which may be called at any time with the command CALL 11011
RICHARD RUSSELL 27 MAR 1988
MIKE FINK OF DOMINO CUBES IS AUTHORIZED TO SELL AND DISTRIBUTE PATCH AND ITS RELATED PROGRAMS