CPU Sequencing 7/20/2019

Instruction Cycles, Machine Cycles and T-states The complete fetching and execution of one instruction is called an instruction cycle. An instruction cycle comprises one or more machine cycles. A machine cycle is a basic operation the processor can perform. Each machine cycle takes a minimum of 3 to a maximum of 6 processor reference clock cycles. In this context a processor clock cycle is called a T-state Instruction cycle – fetching + execution - comprises 1 or more machine cycles Machine cycle – basic operation the processor can perform - takes 3 to max of 6 proc. Ref. clock cylces. T-state - processor clock cycle 7/20/2019

Machine Cycles The 8085A CPU can perform seven basic machine operations. All but the bus-idle cycle involve the transfer of data between the CPU and a peripheral device. The seven machine cycles are : Opcode Fetch fetch the opcode of an instruction from memory Memory Read read data stored at an addressed memory location Memory Write write data to an addressed memory location IO Read read data from an addressed input device IO Write write data to an addressed output device Interrupt Ack acknowledge an interrupt request Bus Idle no bus operation 7/20/2019

Example of an Instruction Cycle Example Instruction : STA addr 7/20/2019

Basic State Diagram The operation of the 8085A can be described with respect to its state diagram ( it is a synchronous state machine ) 7/20/2019

Basic State Diagram Irrespective of the particular machine cycle the processor is performing, the 8085A performs basically the same tasks during each T-state. The Tr-state is the state the machine enters following a hardware reset to the processor. For all machine cycles, other than opcode fetch, the state sequence is T1T2T3T1 For opcode fetch machine cycles, the state sequence is T1T2T3T4T1 or T1T2T3T4T5  T6  T1 depending on the instruction. 7/20/2019

Opcode Fetch Machine Cycle 7/20/2019

Key to Timing Diagrams Indicates a bus. Whilst the lines remain parallel the individual bits of the bus remain unchanged. Indicates a bus. Where the lines cross indicates a possible change in logic level of one or more bits of the bus. Indicates a 0  1 transition of a digital signal Indicates a 1  0 transition of a digital signal Indicates a bus or a bit being in the Hi-Z state ( tri-state ) The tail of the arrow indicates the cause of a signal change. The head of the arrow indicates the affected signal. 7/20/2019

State Processor Functions T1 State The status output pins of the 8085A, S0, S1, IO/M* specify the machine cycle being executed. ( e.g opcode fetch S0 = 1, S1 = 1, IO/M* = 0 ) AD0 - AD7 and A8 - A15 specify the address involved in a data transfer machine cycle (contents of program counter for opcode fetch or memory read the second or third bytes of an instruction) (contents of a CPU register pair, stack pointer register or temporary register pair for other memory read or memory write machine cycles) (contents of a temporary register in IO machine cycles. Note the the same 8-bit address appears on both halfs of the address bus for IO machine cycles) 7/20/2019

State Processor Functions ( Continued) T1 State (cont) Generates an address latch enable (ALE) signal to show valid address information on the multiplexed outputs AD0-AD7. (Valid address is guaranteed on the negative edge of this signal and can be used to externally latch the low byte of the address ) The HALT flip-flop is tested. The HALT flip-flop is internal to the 8085A processor and can be set by a HLT instruction. If it is found set the processor enters the T-halt state after completion of the T1-state, instead of entering the T2-state. ( Note : The T-halt state is not shown on the simplified state diagram) 7/20/2019

State Processor Functions T2 State At the commencement of the T2 state the processor tri-states its multiplexed bus lines AD0-AD7 when executing any read machine cycle. The processor then asserts the RD* control line when executing a read cycle or the WR* control line when executing a write cycle. The processor then places the data to be written to memory or an output device onto the multiplexed bus pins AD0-AD7 for write machine cycles. The processor re-enables the multiplexed bus as an input bus, following the assertion of RD* for read machine cycles. 7/20/2019

State Processor Functions T2 State (cont) The processor samples the input signal RDY. If RDY is set then the processor next enters the T3-state. If RDY is cleared then the processor next enters the Tw-state. ( Not shown on the simplified state transition diagram) The processor samples the input signal HOLD. If the HOLD input is set the processor sets an internal flip-flop, HLDA. If the machine cycle is an opcode fetch machine cycle or if the machine cycle is to read a program byte from memory the program counter is incremented. If the machine cycle is an interrupt acknowledge machine cycle the processor asserts the INTA* control signal instead of RD* 7/20/2019

State Processor Functions T3 State For opcode fetch, memory read or IO read machine cycles the processor deasserts the RD*control signal towards the end of this state. On the rising edge of RD* the data on the lines AD0 - AD7 is latched into the designated processor register ( the instruction register in the case of opcode fetch). For write machine cycles the processor deasserts the control line towards the end of the state and it is incumbent on the external device ( memory or output port ) to use this rising edge to latch the data, placed on the data bus during T2, into the addressed memory or IO location. 7/20/2019

State Processor Functions T3 State (cont) For read cycles the processor disables its bus receivers on AD0 -AD7 following the rising edge of RD*. At the end of the T3-state the processor checks (in the following sequence) : Is T3 the last state in the current machine cycle. If no (opcode fetch) the processor proceeds to the T4-state Is the HLDA flip-flop set. If yes the processor proceeds to the T-hold state ( not shown on the simplified state diagram ) 7/20/2019

State Processor Functions T3 State (cont) Is this machine cycle the last machine cycle in the instruction cycle If no the processor enters the T1-state of the next machine cycle If yes and the internal INTE flip-flop is set, the processor checks its various interrupt inputs and if one is set the processor sets its INTA flip-flop and resets the INTE flip-flop before proceeding to the T1-state of the next machine cycle. 7/20/2019

State Processor Functions T4 State (T5 & T6 states) The processor only enters the T4-state for opcode fetch machine cycles. It uses this state to decode the instruction. For instructions which do not require any further machine cycles for their execution, the processor also uses the T4-state for instruction execution. However, for some single byte 8085A instructions, the single T4-state does not provide sufficient time for instruction decoding and execution. For this class of instruction, the processor uses two more states, T5 and T6 states, for the execution phase. For six T-state opcode fetch machine cycles, the processor re-samples the HOLD input during the T4-state and if asserted sets the HLDA flip-flop. 7/20/2019

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Other States HALT-state The T-halt state is entered after execution of the HLT instruction. The instruction is executed in the T4-state of the opcode fetch machine cycle by the processor setting the internal halt flip-flop. In the T1 state of the next machine cycle the halt flip-flop is tested and when found asserted the processor enters the T-halt state. In the halt state the processor checks the hold input signal and the interrupt inputs (provided interrupts are enabled) If the hold input becomes asserted the processor sets the HLDA flip-flop and then enters the hold state, where it remains until the hold input is deasserted. Following deassertion of the hold input the processor returns to the halt state. 7/20/2019

Other States HALT-state (cont ) If a valid interrupt occurs, when in the halt state, the processor resets the halt flip-flop, sets the inta flip-flop and resets the inte flip-flop ( i.e. disables further interrupts ). The processor then proceeds to the T1-state of the next machine cycle. Note : Only in response to a valid interrupt can the processor permanently leave the halt state. Great care must be taken when including halt instructions in a program. 7/20/2019

Other States Wait-state The processor can enter the T-wait state for all machine cycles except the bus idle machine cycle. If, during the T2-state the processor samples the RDY input in the deasserted state, the processor enters the T-wait state at the end of the T2-state. In the T-wait state all bus and control signals remain as of the end of the T2-state. When in the T-wait state, the processor re-samples the RDY input. If it remains de-asserted another wait state ensues where the process is repeated. If, on the other hand, the RDY input is found asserted the T3-state of the machine cycle is entered. 7/20/2019

Other States 7/20/2019

Need for T-wait States Memory and I/O devices require a finite period of time between receiving address and control signals ( read or write ) and accessing the data at the designated address. Without T-wait states, the processor provides approximately 2.5 master clock cycles between issuing the address and reading the data bus. If the peripheral device requires more than 2.5 clock cycles to access the data, the processor will read invalid data. When writing data, the processor could write data to the incorrect memory or IO location, if sufficient time is not provided. The introduction of T-wait states into the machine cycle increases the time available to the peripheral to read or write data. 7/20/2019

RDY Input Control Circuit to selectively introduce one wait state : 7/20/2019

Other States Hold-state When an external device requests control of the system busses, it does so by asserting the hold input line to the 8085A. The hold input line is sampled by the 8085A in the T2-state, T-4 state and T-halt state and if asserted sets the hlda flip-flop. The hlda flip-flop is tested in the last t-state of any machine cycle and if set the processor enters the T-hold state. The processor remains in the T-hold state whilst the hold input remains asserted. The processor tri-states its bus drivers effectively isolating the processor from the system and thereby allowing an external device to have control of the system busses. When the hold input becomes deasserted, the processor resets the hlda flip-flop, leaves the T-hold state and reassumes execution of machine cycles. 7/20/2019