Datasheet

71M6521DE/DH/FE Data Sheet
Rev 3 Page: 21 of 107
Accumulator (ACC, A): ACC is the accumulator register. Most instructions use the accumulator to hold the
operand. The mnemonics for accumulator-specific instructions refer to accumulator as “A”, not ACC.
B Register: The B register is used during multiply and divide instructions. It can also be used as a scratch-pad
register to hold temporary data.
Program Status Word (PSW):
MSB LSB
CV AC F0 RS1 RS OV - P
Table 9: PSW Register Flags
Bit
Symbol
Function
PSW.7 CV
Carry flag
PSW.6 AC
Auxiliary Carry flag for BCD operations
PSW.5 F0
General purpose Flag 0 available for user.
F0 is not to be confused with the F0 flag in the CESTATUS register.
PSW.4 RS1 Register bank select control bits. The contents of RS1 and RS0 select the working
register bank:
RS1/RS0
Bank selected
Location
00 Bank 0 (0x00 – 0x07)
01 Bank 1 (0x08 – 0x0F)
10 Bank 2 (0x10 – 0x17)
11 Bank 3 (0x18 – 0x1F)
PSW.3 RS0
PSW.2 OV
Overflow flag
PSW.1 -
User defined flag
PSW.0 P
Parity flag, affected by hardware to indicate odd / even number of “one” bits in the
Accumulator, i.e. even parity.
Table 10: PSW Bit Functions
Stack Pointer (SP): The stack pointer is a 1-byte register initialized to 0x07 after reset. This register is incremented
before PUSH and CALL instructions, causing the stack to begin at location 0x08.
Data Pointer: The data pointer (DPTR) is 2 bytes wide. The lower part is DPL, and the highest is DPH. It can be
loaded as two registers (e.g. MOV DPL,#data8). It is generally used to access external code or data space (e.g.
MOVC A,@A+DPTR or MOVX A,@DPTR respectively).
Program Counter: The program counter (PC) is 2 bytes wide initialized to 0x0000 after reset. This register is
incremented when fetching operation code or when operating on data from program memory.
Port Registers: The I/O ports are controlled by Special Function Registers P0, P1, and P2. The contents of the SFR
can be observed on corresponding pins on the chip. Writing a ‘1’ to any of the ports (see Table 11) causes the
corresponding pin to be at high level (V3P3), and writing a ‘0’ causes the corresponding pin to be held at low level
(GND). The data direction registers DIR0, DIR1, and DIR2 define individual pins as input or output pins (see section
Digital I/O for details).