(I have been asked to post the following data about registers and instruction sets.)

Instructions can be categorized according to their method of addressing the hardware registers and/or memory.

         

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.

         

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.

         

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 uses 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.

         

         

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



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.



Combined Addressing Modes:

Some instructions use a combination of addressing modes. A CALL instruction, for example, combines direct addressing and register indirect addressing. The direct address in a CALL instruction specifies the address of the desired subroutine; the register indirect address is the stack pointer. The CALL instruction pushes the current contents of the program counter into the memory location specified by the stack pointer.

Regards,

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