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Four fields in the microinstruction contribute to the new address. CA, 4 bits: Part of the next address, depending on the other fields. CB, 4 bits: Determines bit 1 of the next address. CC, 4 bits: Determines bit 0 of the next address. CD, 2 bits: Controls how the next address is assembled (except when the CB field contains 15).
The output of the IR is available to control circuits, which generate the timing signals that control the various processing elements involved in executing the instruction. In the instruction cycle, the instruction is loaded into the instruction register after the processor fetches it from the memory location pointed to by the program counter.
InstructionJUMP: # To prepare for the next instruction, the instruction-decode microcode has already # moved the program counter to the memory address register. This instruction fetches # the target address of the jump instruction from the memory word following the # jump opcode, by copying from the memory data register to the memory address ...
The program counter (PC), [1] commonly called the instruction pointer (IP) in Intel x86 and Itanium microprocessors, and sometimes called the instruction address register (IAR), [2] [1] the instruction counter, [3] or just part of the instruction sequencer, [4] is a processor register that indicates where a computer is in its program sequence.
The memory controller manages access to memory using the memory bus or a system bus, or through separate control, address, and data buses, to execute the program's commands. The bus managed by the memory controller consists of multiple parallel lines, each representing a binary digit (bit).
When reading from memory, data addressed by MAR is fed into the MDR (memory data register) and then used by the CPU. When writing to memory, the CPU writes data from MDR to the memory location whose address is stored in MAR. MAR, which is found inside the CPU, goes either to the RAM (random-access memory) or cache.
The DEC PDP-10, also 36-bit, had special instructions which allowed memory to be treated as a sequence of fixed-size bit fields or bytes of any size from 1 bit to 36 bits. A one-word sequence descriptor in memory, called a "byte pointer", held the current word address within the sequence, a bit position within a word, and the size of each byte.
The address generation unit (AGU), sometimes also called address computation unit (ACU), [1] is an execution unit inside central processing units (CPUs) that calculates addresses used by the CPU to access main memory. By having address calculations handled by separate circuitry that operates in parallel with the rest of the CPU, the number of ...