Texas Instruments MSP50C6xx manual Top of Stack, TOS, System Registers, Postdecrement

System Registers

or by 2 for double word instructions) each execution cycle and points to the next program memory location to fetch. During a maskable interrupt, the next PC address is stored in the TOS register and is reloaded from TOS after the interrupt encounters an IRET instruction. Call and jump instructions also store the next instruction address by adding PC+2 and then storing the result in the TOS register. Upon encountering a RET instruction, the TOS value is reloaded to the PC. Call instructions may not precede RET instructions. Similarly, a RET instruction may not immediately follow another RET instruction. In these conditions, pipeline operations breaks down and the PC never recovers its re- turn address from the TOS register. The processor stalls, and the only solution is to reset the device. On the other hand, RET can be safely replaced by IRET eliminating processor stalls in all conditions. However, IRET takes one more cycle than RET.

4.2.5Top of Stack, (TOS)

The top of stack (TOS) register holds the value of the stack pointed by the stack register (R7). The MSP50P614/MSP50C614 hardware uses TOS register for very efficient returns from CALL instructions. Figure 4-1 shows the operation of the TOS register. When call instructions are executed, the old TOS register value is pushed into the stack by pre-incrementing R7. The current PC value is incremented by 2 to compute the final return address and is then stored in the TOS register. Thus, the TOS register holds the next PC value pointing to the next instruction. When the subroutine reaches the RET instruction, the program counter (PC) is loaded with the TOS register. Next, the TOS is loaded with the value pointed to by R7. Finally, the stack register (R7) is decremented.

Figure 4–1. Top of Stack (TOS) Register Operation

	Program counter (PC)		Stack register (R7)
		+2	Preincrement
			during write	(+2)
Top of stack register (TOS)
			Postdecrement
	Read before	Increment R7 then
			during read	(+2)
	incrementing R7	store TOS value
	Data memory stack area

The MSP50P614/MSP50C614 development tools use the TOS register for parameter passing. The TOS register must be used with caution inside user programs. If the TOS register and stack register (R7) are not restored to their previous values after using the TOS register in an application, the program can hang the processor or cause the program to behave in an unpredictable way.

Assembly Language Instructions 4-3