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INSTRUCTION FORMATS

 
  
 
Figure 8-2, B.—An example of instructions for a typical microcomputer. INSTRUCTION FORMATS normally  defines  the  location  that  contains  the  operand Instruction formats vary between microprocessors and minicomputers and mainframe computers. As the machine instructions are generally longer in larger computers  with  their  larger  memory  words,  the instruction format or how the instruction is translated differs.  Each  instruction  is  composed  of  fields.  The lengths of instructions and the lengths and positions of the fields differ depending on the instruction and the computer. An operation (function) code is part of all instructions. How the remainder of the instruction is translated and the names assigned to the parts vary. Let’s  take  a  look  at  two  examples  of  computer instruction formats, one for a microcomputer and one for a mainframe. We begin with the op (function) code, which is common to both; only the length differs. A typical machine instruction begins with the specification of an operation to be performed, the operation (op) code. Refer back to figure 8-1. The op code tells the computer/processor what basic operation to perform. The op code, a part of every instruction, is usually  located  at  the  beginning  of  each  instruction format.  Following  the  op  code  is  information,  if needed, to define the location of the data or the operand on  which  the  operation  is  to  be  performed.  This location in memory, called the  operand  address, at the start of the operation (the source), or that will contain the modified operand upon completion of the operation (the destination). The remainder of the instruction and how it is structured differs from one computer or computer type to another. The  designators  in each field and the positions of the fields within the instruction determine how the instruction will affect the operand, registers, memory, and general flow of data in and out of the computer. We discuss the fields and the designators as we discuss the two instruction formats. Microcomputer Instruction Formats A basic 16-bit microinstruction is divided into a number of separate fields. Refer to figure 8-3 as a reference. You’ll notice the lengths of the fields vary. The op code is located in the most significant bits (215 through  213). B (bit 212) tells the computer to use all 16 bits as a word or divide the 16 bits into 8-bit bytes. Figure  8-3.—Example  of  microinstruction  format. 8-7

   


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