Tape Writer

The least significant byte of the register is written to memory first because its location in the register is where sb instruction needs it. The data in the buffer should be in big-endian order, so this (little end) goes into offset 3 from the base address.

Now the remaining bytes of $9 are shifted into the right-most byte one by one and written to memory. Here is a complete version of the program:

## endian.asm
##
## copy $9 to memory in big-endian order
##
## Register Use:
## $8  --- first byte of the tape buffer
## $9  --- 4-byte integer

      .text
      .globl  main

main: 
      lui  $9,0x1234      # put data in $9
      ori  $9,0x5678      #
      lui  $8,0x1000      # $8 is base register
      sb   $9,3($8)       # least significant byte
      srl  $9,$9,8        # move next byte to low order
      sb   $9,2($8)       # bits 8-15 
      srl  $9,$9,8        # move next byte to low order
      sb   $9,1($8)       # bits 16-23 
      srl  $9,$9,8        # move next byte to low order
      sb   $9,0($8)       # most significant byte

      .data
tape:                     # base register points here
      .space 1024         # tape buffer (1K bytes)
         

The .space directive reserves bytes in memory, in this case 1024₁₀ bytes. Pretend this is the buffer from which a tape record will be written. The example program deals with just the first four bytes, but in a real program all 1K bytes of the buffer would be collected before the buffer was written to tape.