Accumulator; B Register; Program Status Word - Siemens C500 User Manual

The arithmetic section of the core performs extensive data manipulation and is comprised of the
arithmetic/logic unit (ALU), an A register, B register and PSW register. Further, it has extensive
facilities for binary and BCD arithmetic and excels in its bit-handling capabilities. Efficient use of
program memory results from an instruction set consisting of 44% one-byte, 41% two-byte, and
15% three-byte instructions. 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, substract, multiply, divide, increment, decrement, BDC-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 as 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 program control section of the core 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 conditional branch logic enables internal and external events to the
processor to cause a change in the program execution sequence.
2.1

Accumulator

ACC is the symbol for the accumulator register. The mnemonics for accumulator-specific
instructions, however, refer to the accumulator simply as A.
2.2

B Register

The B register is used during multiply and divide and serves as both source and destination. For
other instructions it can be treated as another scratch pad register.
2.3

Program Status Word

The Program Status Word (PSW) contains several status bits that reflect the current state of the
CPU. The bits of the PSW are used for different functions which are: two register bank selection bits,
two carry flags and an overflow flag for arithmetic instructions, a parity bit for the content of the ACC,
and two general purpose flags.
The bit definitions of the PSW are shown on the next page.
Semiconductor Group 2-2
CPU Functions
C500 Family
1998-04-01