What machine cycles make up OUT 42H, and how many T-states in total?

Opcode fetch, memory read, I/O write: 4 + 3 + 3 = 10 T-states

Opcode fetch, I/O write: 4 + 3 = 7 T-states

Opcode fetch, memory read, memory write: 4 + 3 + 3 = 10 T-states

Opcode fetch, memory read, memory read, I/O write: 13 T-states

In T1 of the I/O write cycle (M3) of OUT 42H, what is on the address lines?

00H on A15-A8 and 42H on AD7-AD0

20H on A15-A8 and 02H on AD7-AD0 (the next PC value)

42H on A15-A8 and the accumulator content on AD7-AD0

What are the control and status signals during M3 of OUT 42H?

IO/M = 1, S1 = 0, S0 = 1, WR low

IO/M = 0, S1 = 0, S0 = 1, WR low

IO/M = 1, S1 = 1, S0 = 0, RD low

IO/M = 1, S1 = 1, S0 = 1, WR low

When does the output port receive the accumulator content during OUT 42H?

A is put on AD7-AD0 in T2 of M3 with WR low, and the port latches it when WR goes high at the end of T3

In T1 of M3, together with the port address

In T4 of M1, while the opcode is decoded

In T3 of M2, when 42H is read from memory

2071 Bhadra · 8 marks

Bus timing of OUT 42H

Short answer

OUT 42H outputs accumulator to port 42H. It has opcode fetch (4T), memory read of the port address (3T), and I/O write (3T) = 10 T-states. In I/O write, address 42H appears on A15–A8 and AD7–AD0, WR goes low, and accumulator data is sent.

Draw and explain the bus timing for OUT 42H instruction of 8085 microprocessor.

OUT 42H (opcode D3H) sends the content of the accumulator to the output port whose 8-bit address is 42H. It is a 2-byte instruction with 3 machine cycles: opcode fetch (4T), memory read (3T), I/O write (3T) = 10 T-states. Assume it is stored at 2000H, 2001H.

M-cycle	T	A15-A8	AD7-AD0	ALE	Control	IO/M S1 S0
M1 (opcode fetch, 4T)	T1	20H	00H (PCL)	1	-	0 1 1
	T2	20H	Hi-Z (bus floats)	0	RD low	0 1 1
	T3	20H	D3H (opcode in)	0	RD low, rises at end	0 1 1
	T4	-	-	0	none (decode)	0 1 1
M2 (memory read, 3T)	T1	20H	01H	1	-	0 1 0
	T2	20H	Hi-Z (bus floats)	0	RD low	0 1 0
	T3	20H	42H (data in)	0	RD low, rises at end	0 1 0
M3 (I/O write, 3T)	T1	42H	42H	1	-	1 0 1
	T2	42H	A (data out)	0	WR low	1 0 1
	T3	42H	A	0	WR low, rises at end	1 0 1

      |  M1 (OF)  | M2 (MR) | M3 (IOW)|
      |T1 T2 T3 T4|T1 T2 T3 |T1 T2 T3 |
 ALE  |HI lo lo lo|HI lo lo |HI lo lo |
 RD   |hi LO LO hi|hi LO LO |hi hi hi |
 WR   |hi hi hi hi|hi hi hi |hi LO LO |
 IO/M |lo lo lo lo|lo lo lo |HI HI HI |

Explanation

M1 (opcode fetch): address 2000H on the bus, ALE = 1 in T1, IO/M = 0, S1 = S0 = 1. RD goes low in T2-T3 and D3H is read into IR; decoded in T4.
M2 (memory read): address 2001H; RD low; the port address 42H is read into registers W and Z.
M3 (I/O write): IO/M goes high (I/O operation), S1 = 0, S0 = 1. The 8-bit port address 42H is placed on both A15-A8 and AD7-AD0 in T1 (ALE = 1). In T2 the content of A is put on AD7-AD0 and WR goes low; the port latches the data when WR goes high at the end of T3.

Total = 4 + 3 + 3 = 10 T-states. Memory is not accessed in the third cycle because IO/M = 1.

✓ Answer checked by Claude Opus 5.5 (high effort)

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Bus timing of OUT 42H

Explanation

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