Motorola DSP56000 manual Underflow Flag Bit, Reserved Stack Pointer Registration Bits

PROGRAMMING MODEL

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

5.4.5.3Underflow 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.

5.4.5.4Reserved 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.

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