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User Manual

UM001604-0108 Instruction Set

146

IRET changes the value of the Decimal Adjust Flag when the Flag Register saved in the

Stack is restored.

Half Carry Flag

The Half Carry Flag (H) is set to 1 whenever an addition generates a carry bit 3 (Over-

flow) or a subtraction generates a borrow bit 3. The Half Carry Flag is used by the Deci-

mal Adjust (DA) instruction to convert the binary result of a previous addition or

subtraction into the correct decimal (BCD) result. As in the case of the Decimal Adjust

Flag, This Flag is normally not accessed.

IRET changes the value of the Half Carry Flag when the Flag Register saved in the Stack

is restored.

Condition Codes

The C, Z, S, and V Flags control the operation of the ‘Conditional’ Jump instructions. Six-

teen frequently useful functions of the flag settings are encoded in a 4-bit field called the

condition code (cc), which forms bits 4-7 of the conditional instructions.

Condition codes and Flag settings are summarized in Table 34 through Table 36 on page

147. Notation for the Flags and how they are affected are as follows:

Table 34. Z8 Flag Definitions

Flag Description

C Carry Flag

Z Zero Flag

S Sign Flag

VOverflow Flag

D Decimal Adjust Flag

H Half Carry Flag

Table 35. Flag Settings Definitions

Symbol Definition

0 Cleared to 0

1Set to 1

* Set or cleared according to operation

– Unaffected

X Undefined

ZiLOG Z8 Series User Manual

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