8-bit implementation of a recursive fibonacci

fib.S

; Fibonacci
; executable with https://schweigi.github.io/assembler-simulator/
; Result is in register A
JMP .start

; fib(n) <- n is in register A
fib:
  CMP A, 1  ; compare n to a
  JE .fibReturnOne
  JB .fibReturnZero
  PUSH A    ; remember n on the stack
  DEC A     ; n - 1
  CALL fib  ; fib(n - 1)
  MOV B, A  ; remember result of fib(n - 1) in B
  POP A     ; take n from stack and put it in A
  PUSH B    ; put result of fib(n - 1) from B onto stack
  SUB A, 2  ; n - 2
  CALL fib  ; fib(n - 2)
  POP B     ; put result of fib(n - 1) back into B
  ADD A, B  ; fib(n - 1) + fib(n - 2) (in A)
  RET

.fibReturnOne:
  ; result is in register A
  MOV A, 1
  RET

.fibReturnZero:
  ; result is in register A
  MOV A, 0
  RET

.start:
  MOV A, 10
  CALL fib