Radio Shack TRS-80 Service Manual page 30

Color Computer 2 Service Manual
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The CPU has eight data lines (D0-D7). These data lines are
bidirectional and are used by the processor to both route
data to and retrieve data from memory or I/O devices.
The remaining lines on the CPU are used for control
functions, both input control and output control. Of
course, the Vee pin is the power input line to the CPU and
the GND line is the return reference for both power and
signal. The E and Q lines are the clock inputs to the CPU.
These clock signals must be present for the CPU to function.
In the Color Computer 2, these signals are provided by the
SAM chip and are 50% duty cycle clocks at a frequency of
0.89 MHz. As shown in Figure 4-3, Q is a quadrature clock
signal which leads E by 90 degrees.
The CPU contains a number of inputs which serve to initiate
specific sequences of events. The ones used by the Color
Computer 2 are:
RESET* - Used on power up and to reinitialize the CPU.
HALT* - Stops the program flow after the completion
of current instruction. Execution will
continue after HALT is removed.
NMI* - Non-Maskable Interrupt. Always causes the
CPU to "interrupt" its normal program flow
and execute a special "Interrupt Handler"
routine.
IRQ* - Interrupt Request. Similar to NMI but may be
masked (defeated) by setting the I bit in the
CC register.
FIRQ* - Fast Interrupt Request. Similar to IRQ, but
masked by the F bit. It is faster because it
doesn't preserve all registers (as do the
other interrupts).
Upon receipt of the RESET signal, or any of the interrupts
<if enabled), the CPU will get the appropriate subroutine
address from the Vector Table (see the map in the Section
1 - System Description). For the interrupt routines,
registers are preserved on the Stack to be restored upon
receipt of the RTI (Return from Interrupt) instruction.
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