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An ALU has a variety of input and output nets, which are the electrical conductors used to convey digital signals between the ALU and external circuitry. When an ALU is operating, external circuits apply signals to the ALU inputs and, in response, the ALU produces and conveys signals to external circuitry via its outputs.
Another unusual feature is that rather than execute mask, rotate, shift and merge instructions in the arithmetic logic unit (ALU), as is the case with most microprocessors, the 8X300 has separate mask, rotate, shift and merge units. Data can therefore be rotated, masked, modified, shifted and merged (in that order), all in one instruction cycle.
where Z and POS are locations previously set to contain 0 and a positive integer, respectively; Unconditional branching is assured only if Z initially contains 0 (or a value less than the integer stored in POS). A follow-up instruction is required to clear Z after the branching, assuming that the content of Z must be maintained as 0.
For x86 ALU size of 8 bits, an 8-bit two's complement interpretation, the addition operation 11111111 + 11111111 results in 111111110, Carry_Flag set, Sign_Flag set, and Overflow_Flag clear. If 11111111 represents two's complement signed integer −1 ( ADD al,-1 ), then the interpretation of the result is -2 because Overflow_Flag is clear, and ...
Integer overflow can be demonstrated through an odometer overflowing, a mechanical version of the phenomenon. All digits are set to the maximum 9 and the next increment of the white digit causes a cascade of carry-over additions setting all digits to 0, but there is no higher digit (1,000,000s digit) to change to a 1, so the counter resets to zero.
The computation produced as output from the operands is specified by a set of six ordered, single-bit inputs to the ALU. The ALU also emits two single-bit status flags which indicate whether a computation result is zero (zr flag) or negative (ng flag). The CPU also contains two 16-bit registers, labeled D and A. The D (Data) register is a ...
The zero flag is a single bit flag that is a central feature on most conventional CPU architectures (including x86, ARM, PDP-11, 68000, 6502, and numerous others).It is often stored in a dedicated register, typically called status register or flag register, along with other flags.
Support for such operations was an upgrade option rather than being a standard feature. Since the VAX's registers were 32 bits wide, a 128-bit operation used four consecutive registers or four longwords in memory. The ICL 2900 Series provided a 128-bit accumulator, and its instruction set included 128-bit floating-point and packed decimal ...