Machine Control Instructions; Worked 8085 Programs

Machine Control Instructions

Instruction	Function
HLT	Stop execution; buses go high-impedance; waits for interrupt
NOP	No operation — used for timing delays (4T)
EI	Enable maskable interrupts
DI	Disable maskable interrupts
SIM	Set interrupt masks and serial output
RIM	Read interrupt masks and serial input

Worked 8085 Programs

20.1 Program 1 — ADD, CMA, Flag Analysis

MVI D, 8BH ; D = 8BH = 1000 1011B
MVI C, 6FH ; C = 6FH = 0110 1111B
INR C ; C = 70H = 0111 0000B
MOV A, C ; A = 70H
ADD D ; A = 70H + 8BH = FBH
CMA ; A = complement of FBH = 04H
OUT PORT1
HLT

After ADD (A = FBH = 1111 1011B):

Flag	Value	Reason
S	1	MSB = 1 (FBH is "negative" in signed view)
Z	0	Result ≠ 0
AC	0	No nibble carry at bit 3→4
P	0	Odd number of 1s (seven 1s)
CY	0	No carry beyond bit 7

After CMA (A = 04H = 0000 0100B):

Flag	Value	Reason
S	0	MSB = 0
Z	0	Result ≠ 0
P	1	Even number of 1s (one 1 — actually odd; verify: 04H = 00000100 has one 1 → P=0)

Correction for 04H: one 1-bit → odd parity → P = 0

Output on OUT PORT1: 04H

20.2 Program 2 — SUB, ANI, ORA, RRC, CMC

MVI C, 39H ; C = 39H
MVI A, 30H ; A = 30H
DCR C ; C = 38H
SUB C ; A = 30H - 38H = F8H (2's complement)
MVI B, 9FH ; B = 9FH
ANI 80H ; A = F8H AND 80H = 80H
ORA B ; A = 80H OR 9FH = 9FH
RRC ; Rotate right; CY=0 before → A = 4FH, CY = 1
CMC ; Complement carry → CY = 0
OUT PORT1
HLT

After SUB C: A = F8H (borrow occurred → CY = 1) After ANI 80H: A = 80H; P = 0 (one 1-bit), Z = 0 After ORA B: A = 9FH After RRC (with CY=0): A = 4FH, CY = 1 (LSB of 9FH = 1 shifted into carry) After CMC: CY = 0 Final output: A = 4FH

20.3 Program 3 — Conditional Jump (ADI, JC)

MVI A, 8FH ; A = 8FH
ADI 72H ; A = 8FH + 72H = 01H with CY = 1
JC DISPLAY ; Jump because CY = 1
OUT PORT1
HLT
DISPLAY:
XRA A ; A = 00H (XOR A with A)
OUT PORT1
HLT

After ADI 72H: A = 01H, CY = 1 (8FH + 72H = 101H) JC is conditional — taken because CY = 1 XRA A: A = 00H Output displayed: 00H

20.4 Program 4 — LDA, Rotate, XRA

; Memory[2050H] = 73H
LDA 2050H ; A = 73H = 0111 0011B
MOV B, A ; B = 73H
STC ; CY = 1
CMC ; CY = 0
RAR ; Rotate right through carry
XRA B ; XOR A with B
STA 3050H
HLT

After RAR (CY=0 before rotate): LSB of 73H (1) → CY; A = 0011 1001B = 39H After XRA B: A = 39H XOR 73H = 4AH Memory[3050H] = 4AH

20.5 Program 5 — LHLD, XCHG, SHLD

; Memory[2050H] = 3FH, Memory[2051H] = 42H
LXI D, 856FH ; DE = 856FH (not used further in data path)
LHLD 2050H ; L = 3FH, H = 42H → HL = 423FH
XCHG ; Exchange DE ↔ HL; now DE = 423FH, HL = 856FH
SHLD 2050H ; Store HL (856FH) back to 2050H/2051H
HLT

After LHLD: HL = 423FH After XCHG: DE = 423FH, HL = 856FH After SHLD 2050H: Memory[2050H] = 6FH, Memory[2051H] = 85H

20.6 Program 6 — CMP and Flags

MVI A, 50H ; A = 50H
MVI B, 20H ; B = 20H
CMP B ; Compare: 50H vs 20H → A > B
MOV A, B ; A = 20H
HLT

After CMP B: A > B → CY = 0, Z = 0 After MOV A, B: A = 20H (flags unchanged by MOV)

20.7 Program 7 — CMA and 2's Complement via ADI 01H

; Memory[3000H] = 05H
LDA 3000H ; A = 05H
CMA ; A = FAH (1's complement)
STA 3001H ; Store FAH at 3001H
ADI 01H ; A = FAH + 01H = FBH (2's complement of 05H)
STA 3002H
HLT

CMA of 05H: FAH stored at 3001H ADI 01H: FBH (which is the 2's complement of 05H) stored at 3002H