Jonathan A. Titus

Microcomputer Pioneer

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Lesson 6: Instruction Set

by Jonathan A. Titus, David G. Larsen and Peter R. Rony

The MOV and MVI 8080 Instructions
IN THE October 1976 column, we indicated that seven generalpurpose registers (13, C, D, E, H, L, and the accumulator) exist within an 8080 microprocessor chip and that cach register operates on eight bits at a time. These registers are used for many varied purposes, e.g., the storage of an 8-bit constant, an 8bit timing byte, a 16-bit pointer address, 01' an intermediate result from an arithmetic or logical operation.

Let us cxamine thc data transfer instructions MOV D,S in the 8080 instruction set to sec how the generalpurposc registers can be used. We will assume that some data arc initially present within each register. There arc sixty-three different MOV instruclions. each of which speci ries both the SOtlfCC register S of thc data and the destination register D to which thc data are moved. For example, to move data from register E to registcr B, you would use the instruction MClV B, E, which is the Intel mnclllonic notation for the operation of moving data to rcgister B from registcr E. Unfortunately, an 8080bascd microcomputer has no way to understand or interpret a mnemonic instl'llction Stich as MOV Il, n. What is required is the binary representation for MOV n, E: 01000011 2• Th~se eight bits, which comprise the instruction code for MOV n, E, can be manipulated digitally; i.e., they can bc storcd in a semiconductor mcmory device, tmnsmitted over a data bUfi, storcd in an instruction register within the 8080 chip, and dccoded by an i llstruction decoder into a scries of actions that the 8080 performs internally.
Read More: Adobe Acrobat PDF FileThe MOV and MVI 8080 Instructions.pdf [165KB]

The 8080 Logical Instructions
THE CONCEPT of the one important use for multibit logical instructions, such as AND, OR, Exclusive-OR, and COMPLEMENT, has been discussed. I In this column the twenty-eight logical instructions in the 8080A instruction set are summarized. It is very important to note that in the case of each logical instruction, the result is stored in the accumulator. The previous contents of the accumulator is one of the logical variables in the two-variable logical operation, or in the case of the complement instruction, the only logical variable.
Read More: Adobe Acrobat PDF FileThe 8008 Logical Instructions.pdf [49.7KB]

How does a microcomputer make a decision?
ONE of the most important programming characteristics of any digital computer, including a microcomputer, is the ability to make a decision. For a typical microcomputer, we can define a decision as the process of determining further action based upon the logic state of a flag. Aflag is a single flip-flop that can be either set or cleared in response to operations occurring within the microcomputer system. A change of state 0 f the flag is usually an indication either that a particular operation has been completed or that a certain condition exists as a result of a microcomputer operation. Flags can be located either internal or external to the microprocessor chip; the ones that we shall discuss in this column are the internal flags, which are set or cleared in response to specific types of microprocessor instructions such as arithmetic and logical instructions.

The flags that are located within the microprocessor chip are typically associated with the arithmetic- logic unit (AL U], a region within the chip where all arithmetic and logical operations are performed. In the 8080 microprocessor chip, for example, five flags indicate the following conditions:
Read More: Adobe Acrobat PDF FileHow Does A Microcomputer Make A Decision.pdf [37.7KB]