Arithmetic Group Instructions (Part-VI)

DCR R:

• This instruction decrements the contents of specified register by 1.
• The result is stored in the same register.
• Register R can be any general purpose register like A, B, C, D, E, H and L.
• All flags are modified except carry flag.
• It is a one byte instruction.
• Register addressing mode is used.

Example:

If H = 20H

DCR H ; This instruction decrements the contents of H (20H) by one and the result (1FH) is stored in H.

DCR M:

• This instruction decrements the contents of the memory location pointed by HL register pair by 1.
• The result is stored in the same memory location.
• All flags are modified except carry flag.
• It is a one byte instruction.
• Register indirect addressing mode is used.

Example:
If HL = 2000H, (2000H) = 20H

DCR M ; This instruction decrements the contents of location 2000H i.e. 20H by one and the result (1FH) is stored in location 2000H.

DCX Rp:

• This instruction decrements the contents of the specified register pair by 1.
• The result is stored in the same register pair.
• Rp is 16-bit register pair like BC, DE, HL or SP.
• Only higher order register is specified for register pair.
• No flags are modified.
• It is a one byte instruction.
• Register addressing mode is used.

Example:
If BC = 2001H

DCX B ; This instruction decrements the contents of BC register pair (2001H) by one and the result (2000H) is stored in BC.