subr.l | pilAsm

Reading this in 2026. This page reproduces doCar from src64/subr.l from the PicoLisp sources (pil64), as I studied them around 2017. It is the assembly implementation of car, annotated line by line. PicoLisp is MIT-licensed, © Software Lab, Alexander Burger. Since 2020 the current PicoLisp is pil21, which is written in PicoLisp itself and compiled through LLVM, so the line-by-line details here belong to the older 64-bit assembly implementation; the ideas carry over. Current sources are at software-lab.de.

# (car 'var) -> any
(code 'doCar 2)
   push X                         #push whatever value was here onto the stack
   ld X E                         #load pointer to expression E, into X
   ld E ((E CDR))  # Get arg      #load the value
   eval
   num E  # Need variable
   jnz varErrEX
   ld E (E)  # Take CAR
   pop X
   ret

   CPU Registers:

      +---+---+---+---+---+---+---+---+
      |               A           | B |  \\      [A]ccumulator
      +---+---+---+---+---+---+---+---+   D     [B]yte register
      |               C               |  /      [C]ount register
      +---+---+---+---+---+---+---+---+         [D]ouble register
      |               E               |         [E]xpression register
      +---+---+---+---+---+---+---+---+

      +---+---+---+---+---+---+---+---+
      |               X               |         [X] Index register
      +---+---+---+---+---+---+---+---+         [Y] Index register
      |               Y               |         [Z] Index register
      +---+---+---+---+---+---+---+---+
      |               Z               |
      +---+---+---+---+---+---+---+---+

      +---+---+---+---+---+---+---+---+
      |               L               |         [L]ink register
      +---+---+---+---+---+---+---+---+         [S]tack pointer
      |               S               |
      +---+---+---+---+---+---+---+---+

      +-------------------------------+
      |  [z]ero  [s]ign  [x,b,c]arry  |         [F]lags
      +-------------------------------+