MGMT 1050 Chapter Notes - Chapter 4: Instruction Set
MGMT 1050 Chapter 4 Notes – Summary
Introduction
• Many newer machines support 64-bit addresses.
• Further, with additional registers, the simple instruction format shown must be
expanded to handle explicit addressing of multiple registers, including moves between
registers
• As well as identifying the proper register in operations between registers and memory.
• In short, the simple instruction format used in the Little Man Computer is inadequate
for the instruction sets in modern computers.
• The use of instructions of different lengths is one of several techniques developed by
instruction set designers to allow more flexibility in the design of the instruction set.
• Simple instructions can be expressed in a small word, perhaps even a single byte.
• Whereas more complicated instructions will require instruction words many bytes long.
• Longer instructions are stored in successive bytes of memory.
• Thus, a Little Man HALT, IN, or OUT instruction would be stored in a single location.
• A LOAD might require two successive locations to store memory addresses of five digits
or three locations for an eight-digit address.
• The use of variable length instructions is efficient in memory usage, since each
instruction is only as long as it needs to be.
• There are a number of important disadvantages to variable length instructions,
however.
• Most oder oputers irease CPU proessig speed y pipeliig istrutios,
that is, by fetching a new instruction while the previous one is still completing
execution, similar to the processing on an automobile assembly line.
• Variable length instructions complicate pipelining, because the starting point of the new
instruction is not known until the length of the previous instruction has been
determined.
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