Rainbow Electronics W79E8213R - Manual (Page 9)

Preliminary W79E8213/W79E8213R Data Sheet
Publication Release Date: July 11, 2008
- 9 - Revision A2
6.6.1 ALU
The ALU is the heart of the W79E8213 series. It is responsible for the arithmetic and logical functions.
It is also used in decision making, in case of jump instructions, and is also used in calculating jump
addresses. The user cannot directly use the ALU, but the Instruction Decoder reads the op-code,
decodes it, and sequences the data through the ALU and its associated registers to generate the
required result. The ALU mainly uses the ACC which is a special function register (SFR) on the chip.
Another SFR, namely B register is also used in Multiply and Divide instructions. The ALU generates
several status signals which are stored in the Program Status Word register (PSW).
6.6.2 Accumulator
The Accumulator (ACC) is the primary register used in arithmetic, logical and data transfer operations
in the W79E8213 series. Since the Accumulator is directly accessible by the CPU, most of the high
speed instructions make use of the ACC as one argument.
6.6.3 B Register
This is an 8-bit register that is used as the second argument in the MUL and DIV instructions. For all
other instructions it can be used simply as a general purpose register.
6.6.4 Program Status Word
This is an 8-bit SFR that is used to store the status bits of the ALU. It holds the Carry flag, the Auxiliary
Carry flag, General purpose flags, the Register Bank Select, the Overflow flag, and the Parity flag.
6.6.5 Scratch-pad RAM
The W79E8213 series have a 128 bytes on-chip scratch-pad RAM. These can be used by the user for
temporary storage during program execution. A certain section of this RAM is bit addressable, and can
be directly addressed for this purpose.
6.6.6 Stack Pointer
The W79E8213 series have an 8-bit Stack Pointer which points to the top of the Stack. This stack
resides in the Scratch Pad RAM in the W79E8213 series. Hence the size of the stack is limited by the
size of this RAM.
6.7 Power Management
Power Management like the standard 8052, the W79E8213 series also have the IDLE and POWER
DOWN modes of operation. In the IDLE mode, the clock to the CPU is stopped while the timers, serial
ports and interrupt block continue to operate. In the POWER DOWN mode, all clocks are stopped and
the chip operation is completely stopped. This is the lowest power consumption state.