; ==== tests/0500-hello.s ====
   1              ; 0500-hello.s -- Hello World via the UART terminal
   2              ;
   3              ; Polls TX RDY before writing each character, then halts.
   4              ; Run standalone: python sim.py tests/0500-hello.bin --terminal
   5              ;
   6              ; Register usage:
   7              ;   r1  pointer into string (advances each iteration)
   8              ;   r2  current SIXBIT character
   9              ;   r3  TX ready status
  10              ;   r5  TX RDY address (0o7770)
  11              ;   r6  TX BUF address (0o7772)
  12              
  13              %define TXRDY  0o7770
  14              %define TXBUF  0o7772
  15              
  16                      .org 0o1000
  17              
  18  1000  4500          li   r5, TXRDY          ; r5 = TX ready address
      1001  5570
  19  1002  4600          li   r6, TXBUF          ; r6 = TX buffer address
      1003  5672
  20  1004  4110          li   r1, msg            ; r1 = pointer to start of string
      1005  5120
  21              
  22  1006  7213  loop:   lwr  r2, r1             ; load next SIXBIT character
  23  1007  1002          sub  r0, r0, r2         ; T=1 if r2!=0, T=0 if r2=0 (NUL terminator)
  24  1010  4007          bf   done               ; NUL -> done
  25              
  26  1011  7353  txwait: lwr  r3, r5             ; read TX ready status
  27  1012  1003          sub  r0, r0, r3         ; T=1 if ready, T=0 if busy
  28  1013  4076          bf   txwait             ; busy -> keep polling
  29              
  30  1014  7264          swr  r2, r6             ; transmit character
  31  1015  5101          addi r1, 1              ; advance string pointer
  32  1016  5070          bt   loop               ; T=1 (from sub r0,r0,r3) -> next character
  33              
  34  1017  7777  done:   halt
  35              
  36  1020  0010  msg:    .sixbit "HELLO WORLD\n"
      1021  0005
      1022  0014
      1023  0014
      1024  0017
      1025  0040
      1026  0027
      1027  0017
      1030  0022
      1031  0014
      1032  0004
      1033  0037
  37  1034  0000          .word 0                 ; NUL terminator