DP8344 datasheet(150/184 Pages) NSC

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60 Reference Section (Continued)

RETF

Conditional Return

Rcc

Syntax

RETF

f s

Rcc

g rf

(optional syntax)

Affected Flags

If rf e 1 then N Z C and V

Description

Conditionally returns control to the last instruction address

pushed onto the internal Address Stack by popping that ad-

dress into the Program Counter if the state of the flag refer-

enced by f is equal to the state of the bit s or optionally if

the condition cc is met See the tables on the following page

for the flags that f can reference and the conditions that cc

may specify The conditional return instruction also has two

optional operands g and rf The value of g determines if the

Global Interrupt Enable bit GIE is left unchanged (ge0)

restored from the Address Stack (ge1) set (ge2) or

cleared (ge3) If the g operand is omitted then ge0isas-

sumed The second optional operand rf determines if the

ALU flags and register bank selections are left unchanged

(rfe0) or restored from the Address Stack (rfe1) If the rf

operand is omitted then rfe0 is assumed

Example

This example demonstrates both syntaxes of the condition-

al return instruction testing for a carry result from a previous

instruction (ie

C e1) If the condition is met then the

return occurs else the next instruction following the return

is executed The current environment is left unchanged

RETF

001B1

If C e1 then return

Instruction Format

101011110

Opcode

00GIE not affected

01Restore GIE

10Set GIE

11Clear GIE

T-states

2 if condition is not met

3 if condition is met

Bus Timing

Figure 6-1 if condition is not met

Figure 6-2 if condition is met

Operation

If flag f is in state s then

Case g of

0 leave GIE unaffected (default)

1 restore GIE from Address Stack

2 set GIE

3 clear GIE

End case

If rfe1 then

restore ALU flags from Address Stack

restore register bank selection from Address Stack

End if

Address Stack

xPC

End if

Condition Specification Table for ‘‘cc’’

Meaning

Condition Tested for

Zero

Not Zero

Equal

NEQ

Not Equal

Carry

No Carry

Overflow

No Overflow

Negative

Positive

Receiver Active

NRA

Not Receiver Active

Receiver Error

NRE

No Receiver Error

Data Available

DAV

NDA

No Data Available

DAV

TFF

Transmitter FIFO Full

TFF

NTFF

Transmitter FIFO Not Full

TFF

Flag Reference Table for ‘‘f’’

(binary)

Flag Referenced

(000)

in CCR

(001)

in CCR

(010)

in CCR

(011)

in CCR

(100)

in TSR

(101)

in TSR

(110)

DAV

in TSR

(111)

TFF

in TSR

Note

The value of f for DAV differs from the numeric

value for the position of DAV in TSR

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