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The Auxiliary Carry flag is set (to 1) if during an "add" operation there is a carry from the low nibble (lowest four bits) to the high nibble (upper four bits), or a borrow from the high nibble to the low nibble, in the low-order 8-bit portion, during a subtraction. Otherwise, if no such carry or borrow occurs, the flag is cleared or "reset ...
[3] [48] It can perform as an 8-bit 8051, has 24-bit linear addressing, an 8-bit ALU, 8-bit instructions, 16-bit instructions, a limited set of 32-bit instructions, 16 8-bit registers, 16 16-bit registers (8 16-bit registers which do not share space with any 8-bit registers, and 8 16-bit registers which contain 2 8-bit registers per 16-bit ...
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 ...
An 8-bit register can store 2 8 different values. The range of integer values that can be stored in 8 bits depends on the integer representation used. With the two most common representations, the range is 0 through 255 (2 8 − 1) for representation as an binary number, and −128 (−1 × 2 7) through 127 (2 7 − 1) for representation as two's complement.
However, a binary number system with base −2 is also possible. The rightmost bit represents (−2) 0 = +1, the next bit represents (−2) 1 = −2, the next bit (−2) 2 = +4 and so on, with alternating sign. The numbers that can be represented with four bits are shown in the comparison table below.
An eight-bit processor like the Intel 8008 addresses eight bits, but as this is the full width of the accumulator and other registers, this could be considered either byte-addressable or word-addressable. 32-bit x86 processors, which address memory in 8-bit units but have 32-bit general-purpose registers and can operate on 32-bit items with a ...
In the 6502, which has only a single arithmetic register A, this instruction can be represented entirely by its 8-bit opcode $06. [1] On processors with more registers, all that is needed is the opcode and register number, another 4 or 5 bits. On MIPS, for instance, the instruction needs only a 6-bit opcode and a 5-bit register number.
An assembly-language programmer or compiler writer had to be mindful of which operations were possible on each register: Most 8-bit operations could be performed only on the 8-bit accumulator (the A-register), while 16-bit operations could be performed only on the 16-bit pointer/accumulator (the HL-register pair), whereas simple operations ...