MGMT 1050 Lecture Notes - Lecture 20: Memory Address, Opcode, Instruction Set
MGMT 1050 Tutorial 20 Notes – Instruction word formats
Introduction
• Instructions in the Little Man Computer were made up entirely of three-digit decimal
numbers, with a single-digit op code, and a two-digit address field.
• The address field was used in various ways
• For most instructions, the address field contained the two-digit address where data for
the instruction could be found (e.g., LOAD) or was to be placed (STORE).
• In a few instructions, the address field was unused (e.g., HALT).
• For the branch instructions, the address field space was used instead to hold the
address of the next instruction to be executed.
• For the I/O instructions, the address field became a sort of extension of the op code.
• I reality, the I/O address field otaied the address of a I/O deie, i our ase
for the in basket and 02 for the out basket.
• The instruction set in a typical real CPU is similar.
• Again, the instruction word can be divided into an op code and zero or more address
fields.
• A simple 32-bit instruction format with one address field might look like that shown
• In this example, the 32 bits are divided into an 8-bit op code and 24 bits of address field.
• In the Little Man Computer, reference to an address specifically referred to a memory
address.
• However, we have already noted that the computer might have several general-purpose
registers and that it would be necessary for the programmer to select a particular
register to use as a part of the instruction.
• To e ore geeral, e ill use the ord address to refer to ay data loatio,
whether it is a user-accessible register or a memory location.
• We will use the more specific expression memory address when we want to specify that
the address is actually a memory location.
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