Siemens C541U User Manual page 18

Special Function Register PSW (Address D0 H )
Bit No. MSB
D7 H
D0 H CY
Bit
CY
AC
F0
RS1
RS0
OV
F1
P
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.
Stack Pointer
The stack pointer (SP) register is 8 bits wide. It is incremented before data is stored during PUSH
and CALL executions and decremented after data is popped during a POP and RET (RETI)
execution, i.e. it always points to the last valid stack byte. While the stack may reside anywhere in
the on-chip RAM, the stack pointer is initialized to 07 H after a reset. This causes the stack to begin
a location = 08 H above register bank zero. The SP can be read or written under software control.
Semiconductor Group
D6 H D5 H D4 H
AC F0 RS1
Function
Carry Flag
Used by arithmetic instruction.
Auxiliary Carry Flag
Used by instructions which execute BCD operations.
General Purpose Flag
Register Bank select control bits
These bits are used to select one of the four register banks.
RS1 RS0
0 0
0 1
1 0
1 1
Overflow Flag
Used by arithmetic instruction.
General Purpose Flag
Parity Flag
Set/cleared by hardware after each instruction to indicate an odd/even
number of "one" bits in the accumulator, i.e. even parity.
D3 H D2 H
RS0 OV
Function
Bank 0 selected, data address 00 H -07 H
Bank 1 selected, data address 08 H -0F H
Bank 2 selected, data address 10 H -17 H
Bank 3 selected, data address 18 H -1F H
2-3
Fundamental Structure
Reset Value : 00 H
LSB
D1 H D0 H
F1 P PSW
1997-10-01
C541U