Sharp MZ-80B Owner's Manual page 85

76
3.0
TIMING
The Z-80A CPU executes instructions by stepping through a very precise set of a few basic operations. These
include:
Memory read or write
1/0 device read or write
Interrupt acknowledge
All instructions are merely a series of these basic operations. Each of these basic operations can take from three to
six clock periods to complete or they can be lengthened to synchronize the CPU to the speed of external devices. The
basic clock periods are referred to as T cycles and the basic operations are referred to as M (for machine) cycles. Figure
3.0-0 illustrates how a typical instruction will be merely a series of specific MandT cycles. Notice that this instruction
consists of three machine cycles (M 1, M2 and M3). The first machine cycle of any instruction is a fetch cycle which is
four, five or six T cycles long (unless lengthened by the wait signal which will be fully described in the next section).
The fetch cycle (M1) is used to fetch the OP code of the next instruction to be executed. Subsequent machine cycles
move data between the CPU and memory or 1/0 devices and they may have anywhere from three to five T cycles (again
they may be lengthened by wait states to synchronize the external devices to the CPU). The following paragraphs des-
cribe the timing which occurs within any of the basic machine cycles.
T Cyc le
Machin e Cycle
Ml
(OP Code F e tch)
M2
(Me mor y Read )
Instruction Cycl e
M3
(Memory Write )
BASIC CPU TIMING EXAMPLE
FIGURE 3.0-0
All CPU timing can be broken down into a few very simple timing diagrams as shown in Figure 3.0-1 through 3.0-7.
These diagrams show the following basic operations with and without wait states (wait states are added to synchronize
the CPU to slow memory or 1/0 devices).
3 .0-1. Instruction OP code fetch (M 1 cycle)
3.0-2. Memory data read or write cycles
3.0-3 .
1/0 read or write cycles
3.0-4. Bus Request/Acknowledge Cycle
3.0-5. Interrupt Request/ Acknowledge Cycle
3.0-6. Non maskable Interrupt Request/Acknowledge Cycle
3.0-7. Exit from a HALT instruction