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CMX850L8 View Datasheet(PDF) - CML Microsystems Plc

Part Name
Description
MFG CO.
CMX850L8
CML
CML Microsystems Plc CML
'CMX850L8' PDF : 103 Pages View PDF
Communications Controller
CMX850
PSW Register b6: Auxiliary Carry flag (AC)
During ADD, ADDC or SUBB instructions, this bet gets set if there is a carry-out from bit 3, and
cleared otherwise. Gets used by the DAA instruction.
PSW Register b5: General purpose flag 0 (F0)
PSW Register b4-3: Register Bank Select bits (RS1, RS0)
These bits are used to select one of the four register banks.
b4 b3
0 0 Bank 0 selected, R0-7 mapped to hex address $00-$07
0 1 Bank 1 selected, R0-7 mapped to hex address $08-$0F
1 0 Bank 2 selected, R0-7 mapped to hex addresss $10-$17
1 1 Bank 3 selected, R0-7 mapped to hex address $18-$1F
PSW Register b2: Overflow flag (OV)
During ADD, ADDC or SUBB instructions, this bit gets set to indicate the sign of the result (i.e. ACC
bit 7) is incorrect and is cleared otherwise. This assumes that the two operands are signed integers
in the range –128 to 127. More specifically, an overflow occurs if there is a carry-out from bit 6 but
not bit 7, or a carry-out from bit 7 but not bit 6. If the two operands are assumed to be unsigned
integers in the range 0-255, the overflow flag can be ignored.
The overflow flag also gets set by the MUL AB instruction if the product is greater than 255,
otherwise it is cleared. The DIV AB instruction always clears the overflow flag.
PSW Register b12: General purpose flag (F1)
PSW Register b0: Parity flag (P)
Set/cleared by hardware each instruction cycle to indicate an odd/even number of “1” bits in
accumulator A.
1.5.14.3
Stack Point Register (SP)
SP: SFR Address $81
Bits b7-3 cleared to 0 on reset. Bits b2-0 set to 1 on reset.
Bit:
7
6
5
4
3
2
1
0
Bits 7 – 0 of the SP register
The SP register is used as a pointer to the top of the stack area within the 256-byte local RAM.
Temporary program data can be stored and retrieved from the stack using the PUSH and POP
instructions. When data is pushed onto the stack, the stack pointer is incremented before the data
is written. When data is popped off the stack, the stack pointer is decremented after the data is
read. The stack also gets used to automatically store the 2-byte return address for subroutine calls
and interrupt routines: the low-order byte of the return address gets pushed onto the stack first, then
the high order byte.
Stack accesses use indirect addressing to the local RAM, which means that all 256 bytes of the
local RAM can be utilised by the stack.
© 2003 CML Microsystems Plc
59
D/850/6
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