HL = 20FFH. After INX H, what is HL and which flags change?

HL = 2100H; no flags are affected

HL = 2000H; the carry flag is set

HL = 2100H; Z and CY are set because L overflowed

HL = 20FFH + 1 only in L, so HL = 2000H; Z is set

Exchanges L with M[SP] and H with M[SP+1]; SP does not change

Pushes HL on the stack and decrements SP by 2

Which statement about MUL is correct?

MUL does not exist in the 8085; in the 8086 a byte MUL gives AX <- AL x src8 (unsigned)

MUL is an 8085 instruction that multiplies A by B

MUL is an 8086 signed multiply; for a byte operand the result goes in AL only

MUL is an 8086 instruction that always stores a word result in BX

After CMP AX,BX in the 8086, when does JG jump?

When ZF = 0 and SF = OF (signed greater)

When CF = 0 and ZF = 0 (unsigned greater)

2072 Asoj · 4 marks

Short note: INX, XTHL, MUL, JG

Short answer

INX increments a register pair by 1, no flags affected. XTHL exchanges HL with top of stack without altering SP. MUL (8086) performs unsigned multiply, e.g., AX=AL×src; CF/OF set if upper half nonzero. JG (8086) jumps if signed greater, i.e., ZF=0 and SF=OF.

Write short notes on: INX, XTHL, MUL, JG.

INX (Increment register pair) - 8085

INX rp increases the 16-bit content of a register pair (BC, DE, HL or SP) by 1. It is a 1-byte, 6T instruction and no flags are affected. It is used to move pointers through tables.

        LXI H,2000H
        INX H           ; HL = 2001H

XTHL (Exchange top of stack with HL) - 8085

Exchanges L with the byte at M[SP] and H with the byte at M[SP+1]; SP is unchanged and no flags are affected (1 byte, 16T). It is used to replace a value on the stack, for example the return address.

MUL - 8086 (not available in 8085)

The 8085 has no multiply instruction (multiplication is done by repeated addition or shift-and-add). In the 8086, MUL src performs unsigned multiplication:

Byte: AX <- AL x src8
Word: DX:AX <- AX x src16

CF and OF are set if the upper half of the result (AH or DX) is non-zero; other flags are undefined. (IMUL is the signed version.)

        MOV AL,05H
        MOV BL,04H
        MUL BL          ; AX = 0014H

JG - 8086 (not available in 8085)

JG label (same as JNLE) is a conditional jump for signed numbers: it jumps if the first operand is greater than the second after a CMP, that is when ZF = 0 and SF = OF. It is a 2-byte instruction with an 8-bit relative displacement (-128 to +127).

        CMP AX,BX
        JG BIGGER       ; jump if AX > BX (signed)

✓ Answer checked by Claude Opus 5.5 (high effort)

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