Chapter 2 - The Instruction Set

Abstract

The instructions set for the 6812 are classified into seven parts: move instructions, arithmetic instructions, logic instructions, edit instructions, control instructions, input output instructions, and special instructions. Move instructions essentially move 1 or 2 bytes from memory to a register or between registers or memory locations. The computer, as the name implies, is used to compute numerical data or to keep books or control machinery. These operations need arithmetic instructions. Logic instructions are used to set and clear individual bits in A, B, and CCR. They are used by compilers, programs that translate high-level languages to machine code, to manipulate bits to generate machine code. Edit instructions rearrange the bits of data without generating new bits as an ADD does. Large machines have complex edit instructions, but microcomputers have simple ones. The control instructions are those that affect the program counter PC. Control instructions are divided into conditional branching instructions and other control instructions. The input-output (I/O) instruction is easy to describe. With the 6812, a byte is transferred between an accumulator and a register in an I/O device through a memory location chosen by hardware.