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

The first group of registers, RO through R3, establishes the horizontal format and
timing parameters.
Registers R4 through R9 determine vertical format and timing parameters.
The remaining registers, R10 through R17, deal with cursor characteristics, screen
memory addressing, and the light pen interface.
Typically, registers RO through Rll are loaded when the system is tumed on and
should not need to be accessed thereafter. The other six registers, R12 through
R17, will probably need to be accessed on an ongoing basis during display opera-
tions.
R12 and R13 establish the 14-bit starting (top of page) address of screen memory.
The contents of these registers may be manipulated to perform scrolling of the
screen contents.
R14 and R15 establish the 14-bit cursor address which establishes the cursor symbol
position on the screen.
R16 and R17 are only used if a light pen is interhced.

Horizontal Timing and Format Registers

The contents of register RO determines the total time allotted for one scan line
in terms of character dock (CLK) gvJes (total of displayed and undisplayed charac-
ters minus one) per horizontal line, thus determining the horizontal sync (HSYNC)
frequency.
The character row register, Rl, is loaded with the total number of characters minus
one in character dock (CLK) units.
The horizontal sync register, R2, establishes the point at which the HSYNC signal
makes its negative to positive transition, specified in terms of docks. The reference
point for the beginning of the HSYNC pulse is the left-most character position
displayed on the scan line.
R3, the HSYNC width register, uses only the four LSBs of data to establish the
duration of the HSYNC pulse in the range of 1 to 16 character docks. This allows
you to adjust the HSYNC pulse duration to the requirements of your respective
CRT.
Video Graphics Programming
Page 7.23