ST7FOXU0 datasheet(22/123 Pages) STMICROELECTRONICS

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Central processing unit

ST7FOXU0

22/123

5.3.1

Accumulator (A)

The Accumulator is an 8-bit general purpose register used to hold operands and the results

of the arithmetic and logic calculations and to manipulate data.

5.3.2

Index registers (X and Y)

In indexed addressing modes, these 8-bit registers are used to create either effective

addresses or temporary storage areas for data manipulation. (The Cross-Assembler

generates a precede instruction (PRE) to indicate that the following instruction refers to the

Y register.)

The Y register is not affected by the interrupt automatic procedures (not pushed to and

popped from the stack).

5.3.3

Program Counter (PC)

The Program Counter is a 16-bit register containing the address of the next instruction to be

executed by the CPU. It is made of two 8-bit registers PCL (Program Counter low which is

the LSB) and PCH (Program Counter high which is the MSB).

5.3.4

Condition Code register (CC)

The 8-bit Condition Code register contains the interrupt mask and four flags representative

of the result of the instruction just executed. This register can also be handled by the PUSH

and POP instructions.

Reset value: 111x 1xxx

These bits can be individually tested and/or controlled by specific instructions.

Arithmetic management bits

Bit 4 = H Half carry bit

This bit is set by hardware when a carry occurs between bits 3 and 4 of the ALU during

an ADD or ADC instruction. It is reset by hardware during the same instructions.

0: No half carry has occurred.

1: A half carry has occurred.

This bit is tested using the JRH or JRNH instruction. The H bit is useful in BCD

arithmetic subroutines.

111

Read/write

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