Freescale Semiconductor StarCore SC140 - Manual (Page 664)

A-350 SC140 DSP Core Reference Manual
POPN
POPN
Pop a Register from the Software Stack
POPN
Using the Normal Stack Pointer (AGU)
Description
These operations read the memory address pointed to by the normal stack pointer (NSP) into an even or
odd register (De or Do) and adjust NSP regardless of the state of the exception (EXP) bit. All memory
accesses are 32-bit long words. The registers are divided into two groups (even and odd) which determines
the memory offset relative to NSP of the data being read. It is important to pop registers in the same
register grouping by which they were pushed. For example, after the sequence "pushn d1" and "popn d3,"
d3 will hold the data originally in d1. However, "pushn d0" and "popn d1" will not do the same because d0
and d1 are not in the same register group.
One or two POP instructions can appear in an execution set. In both cases, NSP is decremented only once
by 8. When two POP instructions are grouped together in an execution set, each must be in a different
register group.
If the register is a DALU register, bits [39:32] of the destination are sign-extended from bit 31 and the Ln
bit is cleared. Hence, in order to restore a full data register, the extension should be popped last.
Extensions of data registers (with the associated Ln bits) are special. Extensions of even and odd registers
are read from bits [8:0] and [24:16] of the long data word, respectively, both for single register and register
pair operations (see the figure below).
Note: For proper data register restoration, extensions that were pushed as a pair should be popped as a
pair. Extensions pushed as single registers should be popped as single registers.
Operation Assembler Syntax
(NSP – 8)
→ De; NSP – 8 → ΝSP
POPN De
(NSP – 4)
→ Do; NSP – 8 → ΝSP
POPN Do
39 0 16 32
0 16 31 24 8
0 0
EXTENSION
EXTENSION
De (even)
Do (odd)
La +
Lb +
MEMORY LONG WORD
Do De