dsp56800 Freescale Semiconductor, Inc, dsp56800 Datasheet - Page 129

• Manufacturer Part Number: dsp56800
• Description: 16-bit Digital Signal Controllers
• Manufacturer: Freescale Semiconductor, Inc

The effects of the CC bit on the condition codes generated by data ALU arithmetic operations are
discussed in more detail in Section 3.6, “Condition Code Generation,” on page 3-33.
5.1.9.7
The nested looping (NL) bit (OMR bit 15) is used to display the status of program DO looping and the
hardware stack. If this bit is set, then the program is currently in a nested DO loop (that is, two DO loops
are active). If this bit is cleared, then there may be a single or no DO loop active. This bit is necessary for
saving and restoring the contents of the hardware stack, which is described further in Section 8.13,
“Multitasking and the Hardware Stack,” on page 8-34. REP looping does not affect this bit.
It is important that the user never put the processor in the illegal combination specified in Table 5-5. This
can be avoided by ensuring that the LF bit is never cleared when the NL bit is set.
The NL bit is cleared on processor reset. Also see Section 5.1.8.11, “Loop Flag (LF) — Bit 15,” which
discusses the LF bit in the SR.
If both the NL and LF bits are set (that is, two DO loops are active) and a DO instruction is executed, a
hardware-stack-overflow interrupt occurs because there is no more space on the hardware stack to support
a third DO loop.
The NL bit is also affected by any accesses to the hardware stack register. Any MOVE instruction that
writes this register copies the old contents of the LF bit into the NL bit and then sets the LF bit. Any reads
of this register, such as from a MOVE or TSTW instruction, copy the NL bit into the LF bit and then clear
the NL bit.
5.1.9.8
The OMR bits 2, 7, and 9–14 are reserved. They will read as zero during DSC read operations and should
be written as zero to ensure future compatibility.
5.2
The software stack is a last-in-first-out (LIFO) stack of arbitrary depth implemented using memory
locations in the X data memory. It is accessed through the POP instruction and the PUSH instruction
macro (see Section 8.5, “Multiple Value Pushes,” on page 8-19) and will read or write the location in the X
Freescale Semiconductor
Software Stack Operation
Nested Looping Bit (NL) — Bit 15
Reserved OMR Bits — Bits 2, 7 and 9–14
this bit set to one. Otherwise, the chip will not generate the unsigned
conditions correctly.
NL
0
0
1
1
Table 5-5. Looping Status
LF
Program Controller
0
1
0
1
Single DO loop active
Two DO loops active
(Illegal combination)
No DO loops active
DO Loop Status
Software Stack Operation
5-13