Vertical Timing And Format Registers - Zenith Z-100 Reference Manual

Page 7.24
Video Graphics Programming

Vertical Timing and Format Registers

Registers R4 through R9 are normally loaded at system startup and are not nor-
mally changed later. The point of reference for these registers is the top-most
character position displayed on the monitor screen.
The vertical total register, R4, and the vertical sync adjust register determine the
total number of scan line times in a frame, induding vertical retrace, establishing
the overall frame rate or VSYNC frequency.
R4 is a 7-bit register loaded with the total number of character rows. Since a
character row can consist of up to 32 scan lines, it may be difficult for you to
establish a refresh frequency dose to the line frequency. Register R5, a 5-bit VSYNC
adjust register, may then be used to fine tune the VSYNC frequency. R5 is loaded
with a value representing scan line times.
The character rows displayed register, R6, is a 7-bit register which allows you
to select the number of rows of characters to be displayed, up to 128. What you
specify for this register does not determine the position of the VSYNC pulse, but
the point at which display enable (DISPEN) will be reset for vertical retrace.
R7, the verlical sync (VSYNC) position register, determines the point at which
the VSYNC signal makes its negative to positive transition to initiate vertical retrace.
VSYNC position is determined by the character row times, measured from the
first character row on the monitor screen. The VSYNC pulse always has a duration
of 16 scan lines. Since the scan line frequency will vary from one application
to another, and since you cannot change the VSYNC pulse duration, external
circuitry may be needed to achieve a pulse that is compatible with your CRT.
The interlace mode register, RS, determines whether interlaced or noninterlaced
scanning is used.
The value in register R9 determines the number of scan lines per character row.
You may program this 5-bit register for a character row of up to 32 scan lines.
The value you use will dictate the maximum count output by the raster address
signals (RAO through RA4) to the character generator logic. Load R9 with the
desired scan line count minus 1.
The cursor formatting registers are comprised of the cursor start and cursor stop
registers, R10 and Rll; respectively. The five LSBs of each register determine the
scan lines within a character row where the cursor symbol is to be displayed.