Underflow Flag (Bit 5); Reserved Stack Pointer Registration (Bits 6-23); Loop Address Register; Loop Counter Register - Motorola DSP56000 Manual

data (MOVEP) when SSL is specified as a source or destination).
5.4.5.3

Underflow Flag (Bit 5)

The underflow flag is set when a stack underflow occurs. The underflow flag is a "sticky
bit" when the stack error flag is set. That is, when the stack error flag is set, the underflow
flag will not change state. The combination of "underflow=1" and "stack error=0" is an
illegal combination and will not occur unless it is forced by the user. If this condition is
forced by the user, the hardware will correct itself based on the result of the next stack
operation.
5.4.5.4 Reserved Stack Pointer Registration (Bits 6–23)
SP register bits 6 through 23 are reserved for future expansion and will read as zero dur-
ing read operations.

5.4.6 Loop Address Register

The LA is a read/write register which is stacked into the SSH by a DO instruction and is
unstacked by end-of-loop processing or by an ENDDO instruction. The contents of the LA
register indicate the location of the last instruction word in a program loop. When that last
instruction is fetched, the processor checks the contents of the LC register (see the fol-
lowing section). If the contents are not one, the processor decrements the LC and takes
the next instruction from the top of the SS. If the LC is one, the PC is incremented, the
loop flag is restored (pulled from the SS), the SS is purged, the LA and LC registers are
pulled from the SS and restored, and instruction execution continues normally.

5.4.7 Loop Counter Register

The LC register is a special 16-bit counter which specifies the number of times a hardware
program loop shall be repeated. This register is stacked into the SSL by a DO instruction
and unstacked by end-of-loop processing or by execution of an ENDDO instruction. When
the end of a hardware program loop is reached, the contents of the LC register are tested
for one. If the LC is one, the program loop is terminated, and the LC register is loaded with
the previous LC contents stored on the SS. If LC is not one, it is decremented and the
program loop is repeated. The LC can be read under program control, which allows the
number of times a loop will be executed to be monitored/changed dynamically. The LC is
also used in the REP instruction

5.4.8 Programming Model Summary

The complete programming model for the DSP56K central processing module is shown
in Figure 5-9. Programming models for the peripherals are shown in the appropriate user
manuals.
MOTOROLA
PROGRAMMING MODEL
PROGRAM CONTROL UNIT 5 - 17