Intel 8080 manual State Associated Activities, ~2. State Definitions

"data output delay" interval (tOO) following the </>2 clock's leading edge. Data on the bus remains stable throughout the remainder of the machine cycle, until replaced by up- dated status information in the subsequent T 1 state. Observe that a READY signal is necessary for completion of an OUTPUT machine cycle. Unless such an indication is pres- ent, the processor enters the TW state, following the T2 state. Data on the output lines remains stable in the interim, and the processing cycle will not proceed until the READY line again goes high.

The 8080 CPU generates a WR output for the syn- chronization of external transfers, during those machine cycles in which the processor outputs data. These include MEMORY WRITE, STACK WRITE, and OUTPUT. The negative-going leading edge of WR is referenced to the rising edge of the first </>1 clock pulse following T2, and occurs within a brief delay (tDC) of that event. WR remains low until re-triggered by the leading edge of </>1 during the state following T3. Note that any TW states intervening between T2 and T3 of the output machine cycle will neces-

sarily extend WR, in much the same way that DBrN is af- fected during data input operations.

All processor mach ine cycles consist of at least three states: T 1, T2, and T3 as just described. If the processor has to wait for a response from the peripheral or memory with which it is communicating, then the machine cycle may also contain one or more TW states. During the three basic states, data is transferred to or from the processor.

After the T3 state, however, it becomes difficult to generalize. T4 and TS states are available, if the execution of a particular instruction requires them. But not ·all machine cycles make use of these states. It depends upon the kind of instruction being executed, and on the particular machine cycle within the instruction cycle. The processor will termi- nate any mach ine cycle as soon as its processing activities are completed, rather than proceeding through the T4 and TS states every time. Thus the 8080 may exit a machine cycle following the T3, the T41 or the TS state and pro- ceed directly to the T 1 state of the next mach ine cycle.

STATE ASSOCIATED ACTIVITIES

A memory address or I/O device number is placed on the Address Bus (A 15-0); status information is placed on Data Bus (D7-0).

The CPU samples the READY and HOLD in- puts and checks for halt instruction.

TW Processor enters wait state if READY is low

(optional) or if HALT instruction has been executed.

T3An instruction byte (FETCH machine cycle), data byte (MEMORY READ, STACK READ) or interrupt instruction (INTERRUPT machine cycle) is input to the CPU from the Data Bus; or a data byte (MEMORY WRITE, STACK WR ITE or OUTPUT machine cycle) is output onto the data bus.

T4 States T4 and TS are available if the execu-

TStion of a particular instruction requires them;

(optional) if not, the CPU may skip one or both of them. T4 and TS are only used for internal processor operations.

Table 2~2. State Definitions

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