Intel MCS-80/85 manual Iiii, Condition

NOTES:

1.The first memory cycle (M 1) is always an instruction fetch; the first (or only) byte, containing the op code, is fetched during this cycle.

2.If the READY input from memory is not high during T2 of each memory cycle, the processor will enter a wait state (TW) until READY is sampled as high.

3.States T4 and T5 are present, as required, for opera· tions which are completely internal to the CPU. The con- tents of the internal bus during T4 and T5 are available at the data bus; this is designed for testing purposes only. An "X" denotes that the state is present, but is only used for such internal operations as instruction decoding.

4.Only register pairs rp = B (registers B and C) or rp= D (registers D and E) may be specified.

5.These states are skipped.

6.Memory read sub-cycles; an instruction or data word will be read.

7.Memory write sub-cycle.

8.The READY signal is not required during the second and third sub·cycles (M2 and M3). The HOLD signal is accepted during M2 and M3. The SYNC signal is not gene- rated during M2 and M3. During the execution of DAD,

M2 and M3 are required for an internal register-pair add; memory is not referenced.

9.The results of these arithmetic, logical or rotate in- structions are not moved into the accumulator (A) until state T2 of the next instruction cycle. That is, A is loaded while the next instruction is being fetched; this overlapping of operations allows for faster processing.

10.If the value of the least significant 4-bits of the accumu- lator is greater than 9 ~ if the auxiliary carry bit is set, 6 is added to the accumulator. If the value of the most signifi- cant 4-bits of the accumulator is now greater than 9, ~ if the carry bit is set, 6 is added to the most significant 4-bits of the accumulator.

11.This represents the first sub-cycle (the instruction fetch) of the next instruction cycle.

12.If the condition was met, the contents of the register pair WZ are output on the address lines (A()'15) instead of the contents of the program counter (PC).

13.If the condition was not met, sub-cycles M4 and M5 are skipped; the processor instead proceeds immediately to the instruction fetch (Ml) of the next instruction cycle.

14.If the condition was not met, sub-cycles M2 and M3 are skipped; the processor instead proceeds immediately to the instruction fetch (Ml) of the next instruction cycle.

15.Stack read sub-cycle.

16.Stack write sub-cycle.

17. CONDITION		CCC
NZ	not zero (Z = 0)	000
Z	zero (Z = 1)	001
NC	no carry (CY = 0)	010
C	carry (CY = 1)	011
PO	parity odd (P = 0)	100
PE	parity even (P = 1)	101
P	plus (S = 0)	110
M	minus (S = 1)	111

18.I/O sub-cycle: the I/O port's 8-bit select code is dupli- cated on address lines 0-7 (A()'7) and 8-15 (A8-15).

19.Output sub-cycle.

20.The processor will remain idle in the halt state until an interrupt, a reset or a hold is accepted. When a hold re- quest is accepted, the CPU enters the hold mode; after the hold mode is terminated, the processor returns to the halt state. After a reset is accepted, the processor begins execu· tion at memory location zero. After an interrupt is accepted, the processor executes the instruction forced onto the data bus (usually a restart instruction).

SSS or DDD	Value	rp	Value
A	111	B	00
B	000	D	01
C	001	H	10
D	010	SP	11
E	011
H	100
L	101

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