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In a computer's central processing unit (CPU), the accumulator is a register in which intermediate arithmetic logic unit results are stored.. Without a register like an accumulator, it would be necessary to write the result of each calculation (addition, multiplication, shift, etc.) to cache or main memory, perhaps only to be read right back again for use in the next operation.
The hardware unit that performs the operation is known as a multiplier–accumulator (MAC unit); the operation itself is also often called a MAC or a MAD operation. The MAC operation modifies an accumulator a : a ← a + ( b × c ) {\displaystyle a\gets a+(b\times c)}
A sample computer system, the Wombat 1, is provided with CPU Sim.It has the following registers: pc (program counter);acc (accumulator);ir (instruction register);mar (memory address register);
In 1967, Fairchild introduced the first ALU-like device implemented as an integrated circuit, the Fairchild 3800, consisting of an eight-bit arithmetic unit with accumulator. It only supported adds and subtracts but no logic functions. [7] Full integrated-circuit ALUs soon emerged, including four-bit ALUs such as the Am2901 and 74181.
add the contents of a memory cell to the accumulator. 3: TAC: Test accumulator contents performs a sign test on the contents of the accumulator; if minus, jump to a specified memory cell. 4: SFT: Shift shifts the accumulator x places left, then y places right, where x is the upper address digit and y is the lower. 5: OUT: Output
This core memory board was organized in planar fashion as four groups of four banks, each bank carrying two sets of core in a 64 by 64 matrix; thus there were 64 x 64 = 4096 bits per set, x 2 sets giving 8,192 bits, x 4 banks giving 32,768 bits, x 4 groups giving a total of 131,072 bits, and this divided by the machine word size of 16 bits gave ...
Models with more registers (special function registers plus RAM) than fit into the instruction provide multiple banks of memory, and use one of two mechanisms for accessing them: Most baseline core devices, and some mid-range core devices, use the high-order bits of the file select register to select the current register bank.
Here is a simple example of index register use in assembly language pseudo-code that sums a 100 entry array of 4-byte words: Clear_accumulator Load_index 400,index2 //load 4*array size into index register 2 (index2) loop_start : Add_word_to_accumulator array_start,index2 //Add to AC the word at the address (array_start + index2) Branch_and_decrement_if_index_not_zero loop_start,4,index2 //loop ...