Radio Shack TRS-80 Model II Reference Manual page 198

Model II TRSDOS
VDINIT
Video Initialization (function code 7)
Call this initialization routine once before starting any
1;0
to the Display. It
blanks the screen and resets the cursor to the top left corner (position 0 in the
Scroll Mode illustration).
Entry Conditions
B = Character size switch. If B=O then size is set to 40 characters/line.
Otherwise, size is set to 80 characters/line.
C
=
Normal/Reverse switch. If C
=
0 then sets Reverse mode, black on
white background. Otherwise sets Normal mode, white on black
background.
A= 7
Exit Conditions
NZ
=
Error
A
=
Error Code
234