Wedge80 a C128 BASIC extension for VDC graphics
Copyright (c) 2026 Graham (Francesco Gramignani)

Version 1.12
Revision 29/1/2026

https://graham-it.itch.io
https://github.com/graham-it
https://csdb.dk/scener/?id=40810

Introduction

    Wedge80 is a program that extends the Commodore 128's BASIC V7 to support the graphics capabilities of the MOS 8563 VDC (Video Display Controller) chip, which is normally used only in 80-column text mode.
    The name comes from the use of 'wedging', a programming technique that allows the behavior of the BASIC interpreter to be modified transparently to the user.
    Wedge80 is compatible with all official revisions of BASIC V7 ROMs released for the C128, identified by the year of publication (1985 or 1986) displayed on the machine's startup screen.

Extended BASIC

    To access the VDC chip's graphics capabilities, Wedge80 uses the same instruction set as BASIC V7, maintaining the original syntax and extending the range of supported parameters.
    This way, the Wedge80 can manage both the extended graphics on the VDC and the standard graphics on the VIC-II, even simultaneously, ensuring full compatibility with existing software.
    These instructions consist of commands and functions that can be entered by the user in either direct or program mode.

The extended commands are:

GRAPHIC	Selects the current screen mode
SCNCLR	Clears the selected screen mode
COLOR	Sets a color for the selected source
LOCATE	Places the pixel cursor to a specified position
DRAW	Draws dots, lines, and shapes
BOX	Draws a BOX on the screen
CIRCLE	Draws circles, ellipses or arcs
PAINT	Fills an area starting from a specified position
CHAR	Displays a string of characters on current bitmap screen
SSHAPE	Saves an area of the the screen into a BASIC string variable
GSHAPE	Draws a shape stored in a BASIC string variable
AUTO	Enables the AUTOFAST mode
FAST	Sets the clock speed to 2 MHz (then AUTOFAST disabled)
SLOW	Sets the clock speed to 1 MHz (then AUTOFAST disabled)
HELP	Displays Wedge80 info followed by the last BASIC program error
QUIT	Disables Wedge80 BASIC extension

The extended functions are:

RGR(n)	Returns the value of the current screen mode
RDOT(n)	Returns the current bitmap coordinates or pixel status
RCLR(n)	Returns the value of the color source specified
POS(n)	Returns the cursor position within the current text screen window

    A detailed description of the Wedge80's extended instructions is provided in the "BASIC commands" and "BASIC functions" sections at the end of this document (in alphabetical order).
    Instructions not mentioned remain unchanged, and for their use, please refer to the BASIC V7 Encyclopedia, found in the "Commodore 128 Personal Computer System Guide" and the "Commodore 128 Programmer's Reference Guide" (Bantam, 1986).

Installation

    To install Wedge80, after inserting the disk containing the program, you can proceed in the following ways:

    DLOAD "BOOT"        (equivalent to LOAD "BOOT",8)
    RUN

or:

    RUN "BOOT"

    The spaces were added for readability and are ignored by the BASIC interpreter.

    With the VICE C128 emulator, you can drag the provided disk image (.d64) into the program window to automatically load and install Wedge80.

    Alternatively, if you want to keep a BASIC program already present in memory, you can load and execute the machine language code directly, with the command:

    BOOT "WEDGE80"

    This command may not work in early versions of the C128, due to a bug in the KERNAL, so you need to use the following commands:

    BLOAD "WEDGE80"
    SYS 4864

or:

    LOAD "BOOT",8,1
    NEW            (required command)
    SYS 4864

    In the latter case, any BASIC program present in memory will no longer be available.

Utilization

    With Wedge80, you can manage both extended graphics on VDC and standard graphics on VIC-II, even simultaneously.
    Graphics commands are applied to the last graphics mode used, which can be selected with the GRAPHIC command (quickly accessed from the keyboard with the 'F1' key):

-    GRAPHIC 1    VIC-II bitmap (320 x 200, 2 colors per 8 x 8 cell)
-    GRAPHIC 3    VIC-II bitmap (160 x 200, 4 colors per 8 x 8 cell)
-    GRAPHIC 6    VDC bitmap (640 x 200, 2 colors for the entire screen)

    Modes 2 and 4 are similar to modes 1 and 2 with the addition of split screen (not available on VDC).
    To clear the screen, simply add the '1' option preceded by a comma to the commands described above, or use the SCNCLR command (corresponding to the 'F4' key).

    To draw a line on the screen, you can use the DRAW command, which (as in BASIC V7) is based on the Bresenham algorithm:

    DRAW [mode][,x0,y0][TO x1,y1][...]

    The options in square brackets can be omitted, but if the first parameter is missing, the initial coordinate pair must be preceded by a comma.
    The 'mode' option (which replaces 'source', i.e., the color source in the case of the VIC-II) on the VDC screen specifies how pixels should be drawn:

    0 = to delete
    1 = to draw (default)
    2 = performs an XOR with the pixels already present on the screen

    Coordinates represent the starting and ending points of the line, which can be extended with other lines to form a line segment or polygon.
    If the starting coordinates are omitted, the line will start from the last point drawn by the previous instruction. If the 'TO' keyword is missing, one or more isolated points will be drawn.
    In addition, coordinates can be expressed relative to the current position in the following ways:

    - by prefixing the coordinates with the '+' and '-' signs
    - using polar notation, length and angle (in sexagesimal degrees)
      separated by a semicolon ';'

VDC bitmap mode can be deactivated in the following ways:

-    GRAPHIC 5    the cursor switches to the 80-column display
-    GRAPHIC OFF    the cursor remains on the 40-column screen

Auto FAST mode

    When Wedge80 is started, AUTOFAST mode is enabled by default. This allows extended Wedge80 commands to be executed in FAST mode (2 MHz), then automatically reverts to SLOW mode (1 MHz) upon completion.
    AUTOFAST mode works in both direct and program modes and applies to both screens (VDC and VIC-II). It can be deactivated with the standard 'FAST' and 'SLOW' commands.
    To reactivate it, enter the 'AUTO FAST' command (with or without the space between the two keywords).
    At any time, you can find out whether AUTOFAST mode is active by using the 'HELP' command (which can be called up using the relevant hotkey on the C128 keyboard, which on the VICE emulator corresponds to the 'End' key).

ESCAPE sequences

    The C128 offers several functions for controlling the screen editor, accessible directly from the keyboard via the ESC key (which in VICE 3.9 corresponds to the 'F9' key on a PC, or the 'F1' key in previous versions), followed by a key to form a sequence.
    Among these, the 'ESC X' sequence allows you to quickly switch between 40-column and 80-column text mode (and vice versa).
    When the 'ESC X' sequence is detected and the VDC is in bitmap mode, the 80-column text mode will be restored automatically, transparently to the user.

C128 Color table

    The colors available on the VDC chip are different from those of the VIC-II, however in BASIC V7 their codes are rearranged to have a similar correspondence:

Code	VIC-II	VDC
1	Black	Black
2	White	White
3	Red	Dark Red
4	Cyan	Light Cyan
5	Purple	Light Purple
6	Green	Dark Green
7	Blue	Dark Blue
8	Yellow	Light Yellow
9	Orange	Dark Purple
10	Brown	Brown
11	Light Red	Light Red
12	Dark Gray	Dark Cyan
13	Medium Gray	Medium Gray
14	Light Green	Light Green
15	Light Blue	Light Blue
16	Light Gray	Light Gray

Wedge80 Library

    Once installed, Wedge80 offers a function library that allows you to use the VDC graphics routines directly from a program written in machine language.
    This is done via a predefined 'Jump table', ensuring compatibility of programs developed with this library, even if an updated version of Wedge80 is installed.

Jump table:

Routine name	Address	Function (arguments/results)
InstWedge	$1300	Install Wedge80
StartWedge	$1303	Enable Wedge80
QuitWedge	$1306	Disable Wedge80
SetMem80_IO	$1309	Set VDC bitmap address (enabling I/O space)
SetMem80	$130c	Set VDC bitmap address
GetMode80	$130f	Get VDC mode (VDC register 25 -> a)
SwapMode80	$1312	Swap VDC mode (text/bitmap)
SetTxt80	$1315	Activate VDC text mode
SetBmp80	$1318	Activate VDC bitmap mode
ClrBmp80	$131b	Clear VDC bitmap screen
GetBgCol80	$131e	Get VDC background color (RGBI code -> a)
SetBgCol80	$1321	Set VDC background color (a = RGBI code)
GetFgCol80	$1324	Get VDC foreground color (RGBI code -> a)
SetFgCol80	$1327	Set VDC foreground color (a = RGBI code)
GPlot80	$132a	Draw a pixel (plotmode, XPOS, YPOS) (considering WIDTH mode and FILFLG)
GPlot80a	$132d	Draw a pixel (plotmode, XPOS, YPOS)
ReadPt80	$1330	Test a pixel (plotmode, XPOS, YPOS) (cell mask -> a) (zero bit = pixel status) (carry set = coords out of range)
DrawLn80	$1333	Draw a line (plotmode, XPOS, YPOS, XDEST, YDEST)
DrawBox80	$1336	Draw a box (plotmode, XCORDl, YCORDl, XCORD2, YCORD2, BOXANG) (x = fill flag)
Circle80	$1339	Draw a circle (plotmode, XCIRCL, YCIRCL, XRCOS, YRCOS, YRSIN, XRSIN, ANGBEG, ANGEND) (CIRSUB and DSTPOS to get 1st point on circle) (x = degrees per segment)
Paint80	$133c	Fill a graphics area (plotmode, XPOS, YPOS)

BASIC demos

Wedge80 comes with some demo programs written in BASIC to show how it works:

BALLOON	A simple drawing
DARKNESS	Intensive use of lines
GFX EXAMPLES	Examples of geometric shapes
GLOBE80/40	A comparison between the VDC and VIC-II graphics modes
MARBLE	Demonstration of the use of the CIRCLE command
MONOSCOPE	A classic TV test pattern
RND LINES	Draws random lines
RND SHAPES	Draws random polygons (using the CIRCLE command)
SIN(X)	Plots the graph of a mathematical function
RACING CARS	Demonstrates the use of the GSHAPE/SSHAPE commands
SPACESHIPS	Composes a figure by joining multiple modules (GSHAPE/SSHAPE)

Sketch80 demo

    Sketch80 is a simple demonstration program that allows you to draw on the VDC screen in graphical mode using the cursor keys.
    Unlike the previous programs, this program is written in Assembly and uses the graphics library provided by Wedge80, which helps reduce the size of the compiled file.
    The program resides in the BASIC area and can be loaded and started in the usual ways. When the program starts, it checks whether Wedge80 is installed and displays a warning if it is not.

The keyboard commands are as follows:

HOME	Home position (center of the screen)
CLR	Clear screen (SHIFT+HOME)
LEFT	Move left
RIGHT	Move right
UP	Move up
DOWN	Move down
B	Change background color (SHIFT = rapid)
F	Change foreground color (SHIFT = rapid)
C	Clear mode (set/clear)
X	XOR mode (on/off)
D	Pixel width (single/double)
S	Clock speed (FAST/SLOW)
ESC	Exit
RUN/STOP	Exit (with break error)

    The arrow keys are those of the C128's extended keyboard (the gray keys in the top row), which on the VICE emulator correspond to the normal four cursor keys.
    If the edges of the screen (side, top or bottom) are crossed, the cursor will appear on the opposite side.

Technical notes

    The Wedge80 was designed for use on a standard C128 which has only 16 KB of dedicated VDC video RAM, enough to display a 2-color graphics resolution of 640 x 200 pixels.
    Since the VDC's bitmap mode shares some memory with the 80-column text mode, every time you switch between modes, the information in that mode is overwritten.

    In the current version of Wedge80, the BOX command has a bug that prevents the 'fill' option from working correctly in combination with XOR mode.
    However, the XOR mode issues affecting the DRAW and CIRCLE commands in previous versions have been fixed.

    Wedge80 remains active even in case of 'Warm start' (following the RUN/STOP + RESTORE keys are pressed) and the VDC is returned to 80-column text mode.
    If the VDC is in bitmap mode at the time of a 'Warm start', consistency will be lost with the flag that redirects the output of graphics commands to the last used device (VIC-II or VDC).
    In this condition, any command to draw on the VDC will write to the memory in text mode, causing abnormal characters and colors to appear.
    To update the flag indicating the current graphics mode, simply re-enter one of the following commands:

-    GRAPHIC 5    to maintain 80-column text mode
-    GRAPHIC 6    to activate the VDC bitmap mode

    During installation, some of BASIC V7's native graphics routines are copied to RAM and patched to match the VDC chip's bitmap graphics.

The RAM portion hosting the new routines includes:

- Cassette buffer and Disk boot buffer        [$0b00-$0bff]
- RS-232 input buffer                [$0c00-$0cff]
- RS-232 output buffer                [$0d00-$0dff]

    For this reason, while Wedge80 is active, it will not be possible to use the cassette device and the RS-232 interface.

However, the RAM dedicated to sprites remains available:

- Sprite pattern storage area            [$0e00-$0fff]

    Additionally, to access block 1 of RAM, some routines are installed in the 'Common RAM' (specifically in page 3 of the memory):

- Retrieve a character from RAM #1        [$03e4-$03ec] - Write a character to RAM #1            [$03ed-$03f5] - Eval patch routine                [$03f6-$03fb]

    These will overwrite part of the DO_DMA routine, making it impossible to access any external expansion memory (REU):

- RAM-resident portion of DMA routine        [$03f0-$03fc]

    If you ever need to restore the above functions, you can disable Wedge80 with the 'QUIT' command (which in BASIC V7 returns the error message 'UNIMPLEMENTED COMMAND').
    After completing these operations, you can re-enable Wedge80 with the 'SYS 4864' command, unless the 'Application Program Area' memory area ($1300-$1bff) has been overwritten.

BASIC commands

    The options indicated in square brackets are optional and, in the absence of a parameter, the subsequent parameters must be preceded by a comma, in a number equal to the omitted options.
    The commands described here are arranged alphabetically, and spaces between options are added only for readability and are ignored by the BASIC interpreter.

AUTO

Enables the AUTOFAST mode.

Command syntax:

    AUTO FAST (*)

    (*) the space between keywords can be omitted.

Note: without the FAST argument, it operates as the standard AUTO command.

BOX

Draws a BOX on the screen as specified by the parameters that follow the command.

Command syntax:

    BOX [mode],x0,y0 [,x1,y1][,angle][,fill]

mode =
    0    clear pixels
    1    set pixels (default)
    2    XOR with foreground

x0,y0 =        the top left corner coordinates
x1,y1 =        the opposite corner (default = current position)
angle =        the rotation in clockwise degrees (default = 0)

fill =
    0    do not fill the shape (default)
    1    fill the shape (*)

    (*) the fill option does not work with XOR mode, due to the use of the standard BOX routine.

CHAR

Displays a string of characters to the bitmap screen at the position specified.

Command syntax:

    CHAR [mode],col,row [,string][,rev]

mode =
    0    clear using char shape (*)
    1    draw solid char (default)
    2    XOR with foreground (*)
    3    OR with foreground (*)

col =    0..79    column to start printing at
row =    0..24    row to start printing at
string =    string of characters to be printed

rev =
    0    normal mode (default)
    1    (or n > 1) reverse mode

    (*) new options

CIRCLE

Draws circles, ellipses or arcs depending on the parameters specified after the command.

Command syntax:

    CIRCLE [mode][,xc,yc][,xr][,yr][,sa][,ea][,angle][,incr]

mode =
    0    clear pixels
    1    set pixels (default)
    2    XOR with foreground

xc,yc =        the center of the circle (default = current position)
xr =        the horizontal radius (default = 0)
yr =        the vertical radius (default = xr)
sa =        the starting arc angle (default = 0)
ea =        the ending arc angle (default = 360)
angle =        the rotation angle in degrees (default = 0)
incr =        the number of degrees between each segment (default = 2)

Note: varying the number of degrees between each segment, it draws regular polygons as tringle (120), square (90), pentagon (72), exagon (60), etc.

COLOR

Sets a color for the selected source.

Command syntax:

    COLOR src,col

src =
    0    VIC-II background color
    1    VIC-II bitmap foreground color
    2    VIC-II multicolor 1
    3    VIC-II multicolor 2
    4    VIC-II border color
    5    VIC-II or VDC text char color
    6    VDC text/bitmap background color
    7    VDC bitmap foreground color (*)

col =    1..16    the value of the color to be used

    (*) new option

DRAW

Draws dots, lines, and shapes at the positions specified by the coordinates.

Command syntax:

    DRAW [mode][,x0,y0][TO x1,y1][...]

mode =
    0    clear pixels
    1    set pixels (default)
    2    XOR with foreground

x0,y0 =        starting coordinates (default = current position)
x1,y1 =        ending coordinates

Note: without parameters, it draws a dot at current coords.

FAST

Sets the clock speed to 2 MHz.

Command syntax:

    FAST

Note: it implicitly disables the AUTOFAST mode.

GRAPHIC

Selects the screen mode and changes the current graphics screen.

Command syntax:

    GRAPHIC mode[,clr][,sl]

mode =
    0    VIC-II text (40-column)
    1    VIC-II bitmap (320 x 200)
    2    VIC-II split screen bitmap
    3    VIC-II multicolor bitmap (160 x 200)
    4    VIC-II split screen multicolor bitmap
    5    VDC text (80-column)
    6     VDC bitmap (640 x 200) (*)

clr =
    0    do not clear screen (default)
    1    clear screen (implicit on VDC switching from bitmap to text mode)

sl =    0..24    the starting line number of the VIC split screen bitmap (default = 20) (not available on VDC)

Alternate syntax:

    GRAPHIC cmd

cmd =
    CLR    the VIC-II bitmap screen is cleared and then deallocated

    OFF    the VDC display is switched to 80-column text mode and then cleared, without changing the current text mode (*)

    (*) new options

GSHAPE

Draws a shape stored in a BASIC string variable.

Command syntax:

    GSHAPE string[,x,y][,mode]

string =    the string variable where the shape to be drawn is stored

x,y =        the top left corner coordinates where the shape is to be drawn (default = current coords)

mode =
    0    place the shape as is
    1    invert the shape
    2    OR the shape with the area
    3    AND the shape with the area
    4    XOR the shape with the area

HELP

Displays program info, then operates as the standard HELP command.

Command syntax:

    HELP

LOCATE

Places the pixel cursor to a specified position on the bitmap screen.

Command syntax:

    LOCATE x,y

x,y:        the position to move the bitmap pixel cursor to

PAINT

Fills an area starting from a specified position.

Command syntax:

    PAINT [mode][,x,y][,opt]

mode =
    0    clear all contiguous active pixels
    1    fill an empty area (default)

x, y =        the starting coordinates (default = current coords)

opt =
    0    paint an area that is defined by the color source specified
    1    paint an area that is defined by any non-background source (*)

    (*) not applicable to 2-color bitmap

Note: the RUN/STOP key breaks painting at any time.

QUIT

Disables Wedge80 BASIC extension.

Command syntax:

    QUIT

To disable Wedge80, the following steps are performed:
- Restore ESC handler indirect vector
- Restore BASIC indirect vectors
- Restore DO_DMA routine
- Restore VDC text mode
- Clear Wedge80 status flag (INIT_STATUS at $0a04, bit 5)
- Print a message

SCNCLR

Clears the selected screen mode, without changing the current graphics screen.

Command syntax:

    SCNCLR [mode]

mode =
    0    VIC-II text (40-column)
    1    VIC-II bitmap (320 x 200)
    2    VIC-II split screen bitmap
    3    VIC-II multicolor bitmap (160 x 200)
    4    VIC-II split screen multicolor bitmap
    5    VDC text (80-column)
    6    VDC bitmap (640 x 200) (*)

    (*) new option

Note: without an argument, both the current text screen and the current active graphics screen will be cleared.

SLOW

Sets the clock speed to 1 MHz.

Command syntax:

    SLOW

Note: it implicitly disables the AUTOFAST mode.

SSHAPE

Saves an area of the the screen into a BASIC string variable.

Command syntax:

    SSHAPE string,x0,y0 [,x1,y1]

string =    the string variable to which you wish to save the area of screen
        (max 251 bytes = 2008 pixels)
        the remaining 4 bytes are used for the shape size (x,y)

x0,y0 =        the starting corner coordinates for the save
x1,y1 =        the opposite corner (default = current coords)

BASIC functions

    The functions described here accept a numeric value as an argument, enclosed in parentheses, and return the corresponding numeric value.

RCLR

Returns the current color value (1..16) of the color source specified.

Function syntax:

    result = RCLR(n)

    n =
    0    VIC-II background color
    1    VIC-II bitmap foreground color
    2    VIC-II multicolor 1
    3    VIC-II multicolor 2
    4    VIC-II border color
    5    VIC-II or VDC text char color
    6    VDC text/bitmap background color
    7    VDC bitmap foreground color (*)

    (*) new option

RDOT

Returns the current pixel cursor coordinates or the pixel status at current position

depending on the value specified.

Function syntax:

    result = RDOT(n)

    n =
    0    x-coord of the pixel cursor
    1    y-coord of the pixel cursor
    2    pixel status at current position (*)

    (*) result applied to the current graphics screen:
        VIC-II:    color source of the pixel (0..3)
        VDC:    pixel status (0 = off, 1 = on)

RGR

Returns the value of the current screen mode.

Function syntax:

    result = RGR(n)

n =
    0    current screen mode (standard option)
    1    current graphics screen (0 = VIC-II, 1 = VDC) (*)
    2    current width mode (1 = normal, 2 = double) (*)
    3    (or n > 3) current scale mode (0 = not active, 1 = active) (*)

    (*) new options

For n = 0 the values and their meanings are as follows:

    0    40 column text mode
    1    Standard bitmap mode
    2    Split screen bitmap mode
    3    Multicolor bitmap mode
    4    Split screen multicolor bitmap mode
    5    80 column text mode

POS

Returns a value that indicates where the cursor is within the current text screen window (40 or 80 column).

Function syntax:

    result = POS(n)

n  =
    0    cursor column
    1    (or n > 1) cursor row (*)

    (*) new option

Wedge80 v1.12 (c) 2026 Graham

https://graham-it.itch.io
https://github.com/graham-it
https://csdb.dk/scener/?id=40810