Page 20 - DCAP210_INTRODUCTION__TO_MICROPROCESSORS

Introduction to Microprocessors

Notes 1.4.1 Implied Addressing

The addressing mode of certain instructions is implied by the instruction’s function. For example,
the STC (set carry flag) instruction deals only with the carry flag, the DAA (decimal adjust
accumulator) instruction deals with the accumulator.
1.4.2 Register Addressing

Quite a large set of instructions call for register addressing. With these instructions, you must
specify one of the registers A through E, H or L as well as the operation code. With these
instructions, the accumulator is implied as a second operand. For example, the instruction
CMP E may be interpreted as ‘compare the contents of the E register with the contents of the
accumulator.
Most of the instructions that use register addressing deal with 8-bit values. However, a few of
these instructions deal with 16-bit register pairs. For example, the PCHL instruction exchanges
the contents of the program counter with the contents of the H and L registers.

1.4.3 Immediate Addressing
Instructions that use immediate addressing have data assembled as a part of the instruction itself.
For example, the instruction CPI ‘C’ may be interpreted as ‘compare the contents of the accumulator
with the letter C’. When assembled, this instruction has the hexadecimal value FE43. Hexadecimal
43 is the internal representation for the letter C. When this instruction is executed, the processor
fetches the first instruction byte and determines that it must fetch one more byte. The processor
fetches the next byte into one of its internal registers and then performs the compare operation.
Notice that the names of the immediate instructions indicate that they use immediate data. Thus,
the name of an add instruction is ADD; the name of an add immediate instruction is ADI.
All but two of the immediate instructions use the accumulator as an implied operand, as in the
CPI instruction shown previously. The MVI (move immediate) instruction can move its immediate
data to any of the working registers including the accumulator or to memory. Thus, the instruction
MVI D, OFFH moves the hexadecimal value FF to the D register.
The LXI instruction (load register pair immediate) is even more unusual in that its immediate
data is a 16-bit value. This instruction is commonly used to load addresses into a register pair. As
mentioned previously, your program must initialize the stack pointer; LXI is the instruction most
commonly used for this purpose. For example, the instruction LXI SP,3OFFH loads the stack
pointer with the hexadecimal value 30FF.
1.4.4 Direct Addressing

Jump instructions include a 16-bit address as part of the instruction. For example, the instruction
JMP 1000H causes a jump to the hexadecimal address 1000 by replacing the current contents of
the program counter with the new value 1000H.
Instructions that include a direct address require three bytes of storage: one for the instruction
code, and two for the 16-bit address.

1.4.5 Register Indirect Addressing
Register indirect instructions reference memory via a register pair. Thus, the instruction
MOV M, C moves the contents of the C register into the memory address stored in the H and L
register pair. The instruction LDAX B loads the accumulator with the byte of data specified by
the address in the B and C register pair.
1.4.6 Combined Addressing Modes
Some instructions use a combination of addressing modes. A CALL instruction, for example,
combines direct addressing and registers indirect addressing. The direct address in a CALL

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