I have the Dallas Semiconductor ds89c420 with 256bytes of scratch ram. I'm having trouble checking this out, but a quick glance at the memory map on my chip leads me to believe that #0FFh is the last part of indirect ram. So if I used :

MOV SP, #DFh

that would place the stack pointer 32 bytes before the last indirect memory location, freeing up all my registers, as well as giving me around 207 bytes of indirect spacing free for variables later on? Did I catch on correctly?

(forgive me, I have great pains calculating in hex)

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If you are using an 8052 derivative (256 bytes internal RAM), you would initialize the stack pointer using:

    MOV SP,#0FFH-32