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DAA, SPHL, XTHL, PCHL instructions

Short answer

DAA adjusts accumulator after BCD addition by conditionally adding 06H/60H if nibbles >9 or carry. SPHL copies HL to SP. XTHL exchanges HL with top of stack without changing SP. PCHL loads PC with HL for computed jumps.

Explain the following instructions with example program: (i) DAA (ii) SPHL (iii) XTHL (iv) PCHL

(i) DAA: Decimal Adjust Accumulator

After adding two packed BCD numbers with a binary ADD, DAA converts the result in A to a valid BCD result. It adds 06H if the low nibble is greater than 9 or AC = 1, and then adds 60H if the high nibble is greater than 9 or CY = 1. All flags are affected. (1 byte, 4T)

        MVI A,38H       ; BCD 38
        ADI 45H         ; BCD 45 -> A = 7DH (binary sum)
        DAA             ; low nibble D > 9, add 06H -> A = 83H (BCD 83)
        HLT

(ii) SPHL

Copies the content of the HL pair into the stack pointer: SP <- HL. No flags are affected. (1 byte, 6T)

        LXI H,27FFH     ; HL = 27FFH
        SPHL            ; SP = 27FFH (stack initialised)

(iii) XTHL

Exchanges HL with the top of the stack: L <-> M[SP], H <-> M[SP+1]. SP does not change. No flags are affected. (1 byte, 16T)

        LXI SP,3000H
        LXI H,1234H     ; H = 12H, L = 34H
        XTHL            ; if M[3000H]=78H, M[3001H]=56H:
                        ; then HL = 5678H, M[3000H]=34H, M[3001H]=12H

(iv) PCHL

Loads PC with the contents of HL: PC <- HL, so the next instruction is fetched from the address in HL (a computed jump). No flags are affected. (1 byte, 6T)

        LXI H,3000H     ; HL = 3000H
        PCHL            ; PC = 3000H, execution continues at 3000H

✓ Answer checked by Claude Opus 5.5 (high effort)

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