TIA 1A - TELEVISION INTERFACE ADAPTOR (MODEL 1A)
5. Horizontal Motion Registers
A. General Description
There are five write only registers on this chip that
contain the horizontal motion values for each of the five
moving objects. A typical horizontal motion register is
shown in figure 4 . The data bus (bits 4 through 7) is
written into these addresses (HMP0, HMPl, HMM0, HMMl, HMBL)
to load these registers with motion values. These registers
supply extra (or fewer) clocks to the horizontal position
counters only when commanded to do so by an HMOVE address
from the microprocessor. These registers may all be cleared
to zero (no motion) simultaneously by an HMCLR command from
the microprocessor, or individually by loading zeros into
each register. These registers are each four bits in
length and may be loaded with positive (left motion),
negative (right motion) or zero (no motion) values.
Negative values are represented in twos complement format.
B. Timing constraints
These registers may be loaded or cleared at almost any
time. The motion values they contain will be used only when
an HMOVE command is addressed, and then all five motion
values will be used simultaneously into all five horizontal
position counters once. The only timing constraint on this
operation involves the HMOVE command. The HMOVE command
must be located in the microprocessor program immediately
after a wait for sync (WSYNC) command. This assures that
the HMOVE operation begins at the leading edge of
horizontal blank, and has the full blank time to supply
extra or fewer clocks to the horizontal position counters.
These registers should not be modified for at least 24
Computer cycles after the HMOVE command.