Page 151 - DCAP210_INTRODUCTION__TO_MICROPROCESSORS

Unit 10: Programming Techniques with Additional Instructions

Notes
DCR M Decrements memory contents
(a) It is a 1-byte instruction
(b) Decrements the contents of memory by 1, not
the memory address
(c) (HL) register specifies the memory location.

INR and DCR instructions affect all the flags except the carry (CY) flag. As all these instructions
involve a memory location and a register and so uses register indirect addressing more.

Example: Write instructions to add the contents of the memory location 8000H to (A) and
subtract the contents of the memory 8001H from the first sum. Assume the Accumulator has 68H
the memory location 8000H has 30H, and the location 8001H has 7FH.
Solution: The contents of HL pair 8000H specify the memory location. The instruction ADD M
adds 30H the contents of memory location 8000H, to the contents of the Accumulator (68H) the
instruction INX H points to the next memory location 8001H and the instruction SUB M subtracts
the contents (7FH) of memory location 8001 from the previous sum. The assembly language
program is as follows:
Machine code Opcode Operands
(Hex)
21 LXI H, 8000H
00
80
86 ADD M
23 INX H
96 SUB M

10.3.2 Related to 16-bits (Register Pairs)
The instructions related to incrementing/ decrementing 16-bit contents in a register pair are listed
in Table 10.5. These instructions do not affect any of the status of flags.

Table 10.5: List of Arithmetic Operations Related to 16-bit.

Opcode Operands Description

INX R Increments register pairs
INX B (a) It is a 1-byte instruction.
INX D (b) Assumes the contents of two registers as one
INX H 16-bit number
INX SP (c) Increments the contents by 1
DCX R Decrements register pairs
DCX B (a) It is a 1-byte instruction
DCX D (b) Assumes the contents of two registers as one
DCX H 16-bit number.
DCX SP (c) Decreases the contents of registers pair by 1

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