I don't know what would be the equivalent in SDCC syntax, might be as simple as
oldSP = $+1Generally, what this pseudo-op construct means is: "assign a label (oldSP) to the current address ($) + 1". So in this example, oldSP will point to the address immediately after the "ld sp,nnnn" instruction, which happens to be the "nnnn" part. There is some other part in the code (usually in the init part) that writes the value of SP at that point in time to the location of oldSP. The snippet "ld sp,old_value" is then usually called on exit, to restore SP to it's proper value, as many beeper engines mess with the stack a lot.
Anyway, the point is that if you don't have any form of "label equ $+x" available, you can also simply locate all points in the code that write to 'label' (in case of the SP backup there'll usually just be one such location). That write op will look something like
ld (oldSP),spwhich you can change to
ld (oldSP + 1),spand then change the "oldSP equ $+1" to simply "oldSP".