RM0312 datasheet(21/275 Pages) STMICROELECTRONICS

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RM0312

Central processing unit (CPU)

•

H: Half carry bit

The H bit is set to 1 when a carry occurs between the bits 3 and 4 of the ALU during an ADD

or ADC instruction. The H bit is useful in BCD arithmetic subroutines.

•

I0: Interrupt mask level 0

See Flag I1.

•

N: Negative

When set to 1, this bit indicates that the result of the last arithmetic, logical or data

manipulation is negative (i.e. the most significant bit is a logic 1).

•

Z: Zero

When set to 1, this bit indicates that the result of the last arithmetic, logical or data

manipulation is zero.

•

C: Carry

When set, C indicates that a carry or borrow out of the ALU occurred during the last

arithmetic operation on the MSB operation result bit. This bit is also affected during bit test,

branch, shift, rotate and load instructions. See the ADD, ADC, SUB, and SBC instructions.

In a division operation, C indicates if trouble occurred during execution (quotient overflow or

zero division). See the DIV instruction.

In bit test operations, C is the copy of the tested bit. See the BTJF and BTJT instructions.

In shift and rotate operations, the carry is updated. See the RRC, RLC, SRL, SLL, and SRA

instructions.

This bit can be set, reset or complemented by software using the SCF, RCF, and CCF

instructions.

Example: Addition

$B5 + $94 = "C" + $49 = $149

Table 2. Interrupt levels

Interruptability

Priority

Interruptable main

Lowest

Highest

Interruptable level 1

Interruptable level 2

Non interruptable

1011

010

1001

010

0100

100

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