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      The stack is 16 bits wide and four (TMS32020) or eight (TMS320C25)
      levels deep. The PC stack is accessible through the use of the PUSH
      and POP instructions. Whenever the contents of the PC are pushed
      onto the top of the stack, the previous contents of each level are
      pushed down, and the bottom (fourth/eighth) location of the stack
      is lost. Therefore, data will be lost if more than four/eight
      successive pushes occur before a pop. The reverse happens on pop
      operations. Any pop after three/seven sequential pops yields the
      value at the bottom stack level. All of the stack levels then
      contain the same value. Two additional instructions, PSHD and POPD,
      push a data memory value onto the stack or pop a value from the
      stack to data memory. These instructions allow a stack to be built
      in data memory for the nesting of subroutines/interrupts beyond
      four/eight levels.

      Note that on the TMS32020, the TBLR/TBLW, MAC/MACD, and BLKD/BLKP
      instructions use one level of the stack. The TMS320C25 contains a
      separate stack for use with these instructions, and no level of the
      PC is used.