UPSD325X datasheet(18/176 Pages) STMICROELECTRONICS

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µPSD325X DEVICES

18/176

Accumulator. The Accumulator is the 8-bit gen-

eral purpose register, used for data operation such

as transfer, temporary saving, and conditional

tests. The Accumulator can be used as a 16-bit

Figure 7. Configuration of BA 16-bit Registers

B Register. The B Register is the 8-bit general

purpose register, used for an arithmetic operation

such as multiply, division with Accumulator

Stack Pointer. The Stack Pointer Register is 8

bits wide. It is incremented before data is stored

during PUSH and CALL executions. While the

stack may reside anywhere in on-chip RAM, the

Stack Pointer is initialized to 07h after reset. This

causes the stack to begin at location 08h.

Figure 8. Stack Pointer

Program Counter. The Program Counter is a 16-

bit wide which consists of two 8-bit registers, PCH

and PCL. This counter indicates the address of the

next instruction to be executed. In RESET state,

the program counter has reset routine address

(PCH:00h, PCL:00h).

Program Status Word. The

Program

Status

Word (PSW) contains several bits that reflect the

current state of the CPU and select Internal RAM

(00h to 1Fh: Bank0 to Bank3). The PSW is de-

scribed in Figure 9, page 19. It contains the Carry

flag, the Auxiliary carry flag, the Half Carry (for

BCD operation), the general purpose flag, the

Parity flag.

[Carry Flag, CY]. This flag stores any carry or not

borrow from the ALU of CPU after an arithmetic

operation and is also changed by the Shift Instruc-

tion or Rotate Instruction.

[Auxiliary Carry Flag, AC]. After operation, this is

set when there is a carry from Bit 3 of ALU or there

is no borrow from Bit 4 of ALU.

[Register Bank Select Flags, RS0, RS1]. This flags

select

one

four

bank(00~07H:bank0,

08~0Fh:bank1, 10~17h:bank2, 17~1Fh:bank3) in

Internal RAM.

[Overflow Flag, OV]. This flag is set to ’1’ when an

overflow occurs as the result of an arithmetic oper-

ation involving signs. An overflow occurs when the

result of an addition or subtraction exceeds +127

(7Fh) or -128 (80h). The CLRV instruction clears

the overflow flag. There is no set instruction. When

the BIT instruction is executed, Bit 6 of memory is

copied to this flag.

[Parity Flag, P]. This flag reflect on number of Ac-

cumulator’s 1. If number of Accumulator’s 1 is odd,

P=0. otherwise P=1. Sum of adding Accumulator’s

1 to P is always even.

R0~R7. General purpose 8-bit registers that are

locked in the lower portion of internal data area.

Data Pointer Register. Data Pointer Register is

16-bit wide which consists of two-8bit registers,

DPH and DPL. This register is used as a data

pointer for the data transmission with external data

memory in the PSD Module.

AI06637

Two 8-bit Registers can be used as a "BA" 16-bit Registers

AI06638

SP (Stack Pointer) could be in 00h-FFh

00h

Stack Area (30h-FFh)

00h-FFh

Hardware Fixed

Bit 15

Bit 0

Bit 8 Bit 7

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