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• 16-bit data (words have 16 bits)
• 16-bit instructions, 4 for the opcode and 12 for the address
• A16-bit accumulator (AC)
• A16-bit instruction register (IR)
• A16-bit memory buffer register (MBR)
• A12-bit program counter (PC)
• A12-bit memory address register (MAR)
• An 8-bit input register
• An 8-bit output register
Figure 4.8 shows the architecture for MARIE.
Before we continue, we need to stress one important point about memory. In
Chapter 8, we presented a simple memory built using D flip-flops. We emphasize
again that each location in memory has a unique address (represented in binary)
and each location can hold a value. These notions of the address versus what is
actually stored at that address tend to be confusing. To help avoid confusion, just
visualize a post office. There are post office boxes with various “addresses” or
numbers. Inside the post office box, there is mail. To get the mail, the number of
the post office box must be known. The same is true for data or instructions that
need to be fetched from memory. The contents of any memory address are manipulated
by specifying the address of that memory location. We shall see that there
are many different ways to specify this address.
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