SAB80515 Datasheet, PDF (16/270 Pages) Siemens Semiconductor Group – 8-Bit Single-Chip Microcontroller Family

Central Processing Unit
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Central Processing Unit
3.1 General Description
The CPU (Central Processing Unit) of the SAB 80(C)515 consists of the instruction decoder, the
arithmetic section and the program control section. Each program instruction is decoded by the
instruction decoder. This unit generates the internal signals controlling the functions of the individual
units within the CPU. They have an effect on the source and destination of data transfers, and
control the ALU processing.
The arithmetic section of the processor performs extensive data manipulation and is comprised of
the Arithmetic/Logic Unit (ALU), an A register, B register and PSW register. The ALU accepts 8-bit
data words from one or two sources and generates an 8-bit result under the control of the instruction
decoder. The ALU performs the arithmetic operations add, subtract, multiply, divide, increment,
decrement, BCD-decimal-add-adjust and compare, and the logic operations AND, OR, Exclusive
OR, complement and rotate (right, left or swap nibble (left four)). Also included is a Boolean
processor performing the bit operations of set, clear, complement, jump-if-not-set, jump-if-set-and-
clear and move to/from carry. Between any addressable bit (or its complement) and the carry flag,
it can perform the bit operations of logical AND or logical OR with the result returned to the carry
flag. The A, B and PSW registers are described in section 4.4.
The program control section controls the sequence in which the instructions stored in program
memory are executed. The 16-bit program counter (PC) holds the address of the next instruction to
be executed. The PC is manipulated by the control transfer instructions listed in the chapter
"Instruction Set". The conditional branch logic enables internal and external events to the processor
to cause a change in the program execution sequence.
Semiconductor Group
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