MGMT 1000 Chapter Notes - Chapter 18: Memory Buffer Register, Opcode, Maximum Capacity
MGMT 1000 Chapter 18 Notes – Summary
Introduction
• The original IBM PC came supplied with 64 KB of memory, with a maximum capacity of
640 KB.
• In fact, Bill Gates, of Microsoft, was quoted at the time as saying that he could see no
need for more than 640 KB of memory, ever!
• The size of the data word to be retrieved or stored in a single operation is determined
by the size of the memory data register and by the width of the connection between
memory and the CPU.
• In most modern computers, data and instructions found in memory are addressed in
multiples of 8-bit bytes.
• This establishes the minimum instruction size as 8 bits.
• Most instructions cannot fit practically into 8 bits. If one were to allow 3 bits for the op
code (eight instruction types), only 5 bits remain for addressing.
• Five bits allows 25 = 32 different addresses, which is clearly insufficient address space.
• As a result, longer instructions of 16, 24, 32, or even more bits will be stored in
successive memory locations.
• In the interest of speed, it is generally desirable to retrieve an entire instruction with a
single fetch, if possible.
• Additionally, data to be used in arithmetic calculations frequently requires the precision
of several bytes.
• Therefore, most modern computer memories are designed to allow the retrieval or
storage of at least 4 and, more commonly, 8 or even 16, successive bytes in a single
operation.
• Thus, the memory data register is usually designed to retrieve the data or instruction(s)
from a sequence of several successive addresses all at once.
• The MDR will be several bytes wide section(s) from a sequence of several successive
addresses all at once, and the MDR will be several bytes wide.
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