With 16 bits, a maximum of 2 16 = 65,536 words can be addressed. For example, with a 16-bit address register, the address bus has 16 bits to address RAM and ROM. And that limits the amount of memory that can be accessed.
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It is important to note that the PC and the MAR (address buffer) have a fixed length of so many bits. Program counter identifies the address in memory to be accessed. The instructions in a program are stored in sequential memory locations.įigure 6.7. After the instruction is executed, the PC is incremented (add 1) to the next address in sequence, or 44. The address in the PC is applied to the memory, causing the instruction in location 43 to be fetched and executed. If a program begins with an instruction stored in memory location 43, the PC is first loaded with the address 43. It stores a binary word that is used as the address for accessing the instructions in a program. The PC is really a counter and a register. The CPU decodes the instruction, and then executes it.
#EBOLAXTERNL MEMORY NOTE 4 CODE#
The instruction word is also called the op code or operation code.
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The instruction is a binary word or code that defines a specific operation to be performed. When the CPU fetches an instruction from memory, it is temporarily stored in the IR. The IR is used to store the instruction word. Frenzel Jr., in Electronics Explained (Second Edition), 2018 Control UnitĬPUs also have several other registers, including the instruction register (IR) the program counter (PC), also called the instruction counter and the memory address register (MAR), also called the address buffer. 14 // Set up the initial variables and resetġ6 $ display (” time ∖ t clk reset inc load valid data ”) ġ7 $ monitor (” % g ∖ t % b % b % b % b % b ”,ġ8 $ time, clk, nrst, mar_load, data, address) Ģ0 nrst = 1 // set the reset to 1 (not reset)Ģ3 #5 nrst = 0 // reset = 0 : resets the counterĢ4 #10 nrst = 1 // reset back to 1 : counter can startĢ8 #200 $ finish // Finish the simulationģ3 #5 clk = ˜clk // Clock every 5 time slotsģ9 mar DUT (clk,nrst,mar_load,data,address)