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106 www.xilinx.com PicoBlaze 8-bit Embedded Microcontroller
UG129 (v1.1.2) June 24, 2008
Appendix : PicoBlaze Instruction Set and Event Reference
R
RETURN [Condition] — Return from Subroutine Call, Possibly with
Conditions
The RETURN instruction is the complement to the CALL instruction. The RETURN
instruction is also conditional. The new PC value is formed internally by incrementing the
last value on the program address stack, ensuring that the program executes the
instruction following the CALL instruction that called the subroutine. The RETURN
instruction has no effect on the status of the flags.
The RETURN instruction has both conditional and unconditional variants. A conditional
RETURN is only performed if a test performed against either the ZERO flag or CARRY flag
is true. If unconditional or if the condition is true, the RETURN instruction pops the return
address from the top of the CALL/RETURN stack into the PC. The popped value forces
the program to return to the instruction immediately following the original subroutine
CALL.
Ensure that a RETURN is only performed in response to a previous CALL instruction so
that the CALL/RETURN stack contains a valid address.
The RETURN instruction does not affect the ZERO or CARRY flags. The flag values set prior
to the RETURN instruction are maintained and available after the return from the
subroutine call.
Condition
Depending on the specified condition, the program returns from a subroutine call. If the
specified condition is not met, the program continues on to the next instruction.
Pseudocode
if (Condition = TRUE) then
; pop the top of the CALL/RETURN stack into PC
; TOS = Top of Stack
PC Å TOS + 1; incremented value from Top of Stack
else
PC Å PC + 1
endif
Registers/Flags Altered
Registers: PC, CALL/RETURN stack
Flags: Not affected
Notes
Do not use the RETURN instruction to return from an interrupt. Instead, use the RETURNI
instruction.
Table C-4: RETURN Instruction Conditions
Condition Description
<none> Always true. Return from called subroutine unconditionally.
C CARRY = 1. Return from called subroutine if CARRY flag is set.
NC CARRY = 0. Return from called subroutine if CARRY flag is cleared.
Z ZERO = 1. Return from called subroutine if ZERO flag is set.
NZ ZERO = 0. Return from called subroutine if ZERO flag is cleared.