GCOL

GCOL is used to select the logical colours used by graphics statements.

GCOL

Availability	Present in all original versions of BBC BASIC.
Syntax	BASIC I-IV	GCOL <numeric>,<numeric>
	BASIC V	GCOL <numeric>,<numeric> [TINT <numeric>]
Token (hex)	BASIC I-V	E6 (statement)
Description	BASIC I-V	If bit 7 of the second <numeric> is 0 then GCOL sets the foreground graphics paint mode to the first <numeric> and the foreground graphics colour to the second <numeric> modulo 128. If bit 7 of the second <numeric> is 1 then GCOL sets the colour and paint mode of background graphics instead. The current contents of the screen are not affected. In BASIC V the TINT argument sets the foreground or background tint.
Associated keywords	CLG, CLS, COLOUR, MODE, PLOT

Description

This statement sets the colour to be used by all subsequent graphics operations. It can set the foreground and background graphics colours. It also specifies how the colour is to be placed on the screen.

The first number specifies the mode of action.

• 0 plot the specified colour
• 1 OR the colour with the colour already there
• 2 AND the colour with the colour already there
• 3 XOR (exclusive-OR) the colour with the colour already there
• 4 NOT (invert) the colour already there
• 5 no change to colour already there (not initially documented)
• 6 clear the colour - set it to colour 0 (not initially documented)
• 7 various actions resulting from a table overflow

With the Graphics Extension ROM, the Master and later:

• n*16+k - select fill pattern n for n=1,2,3,4

On Arthur and later:

• 6 sets the pixel to (NOT colour AND the colour already there)
• 7 sets the pixel to (NOT colour OR the colour already there)
• 8+k - fill pattern uses a transparent background
• 5*16+k - select a fill pattern made from all four fill patterns in a block
• 6*16+k - select a large fill pattern
• 7*16+k - select a large fill pattern

The second number specifies the logical colour. If the number is less than 128 then it defines the graphics foreground colour. If the number is greater than 127 then it defines the graphics background colour.

GCOL simply sends VDU 18,action,colour. Some versions of BASIC allow GCOL <colour> which sends VDU 18,0,colour.

The BBC VDU driver does no range checking on the GCOL action, so using out-of-range actions can cause odd effects.

Extensions

The defined GCOL actions are &00-&7F in blocks of 16:

• &00+x plot action
• &10+x fill pattern 1
• &20+x fill pattern 2
• &30+x fill pattern 3
• &40+x fill pattern 4
• &50+x super fill pattern 1-4
• &60+x large fill pattern
• &70+x large fill pattern

GCOL actions &80-&FF are undefined, and selecting them will result in undefined behaviour. Some applications have taken advantage of these unused actions to intercept OSWRCH to have a useful one-byte VDU command sequence.

• &89 (also GCOL -ASC"w") - select a window
• &8C (also GCOL -ASC"t") - select a text style
• &8D (also GCOL -ASC"s") - select a text style
• &A0+x reserved
• &E0+x application use
• &F0+x application use