Sunday, 18 January 2009

Hello World for the RSSB Virtual Computer

The RSSB virtual computer has a single instruction, reverse subtract and skip if borrow. Each instruction has one operand, a pointer into memory. RSSB subtracts the accumulator from the contents of memory and the result is stored in both. If the accumulator was greater than the number in memory, the next instruction is skipped.

Jumps can be implemented by manipulating the instruction pointer at memory location 0, which normally requires 4 instructions. A conditional jump requires 6 instructions. Other special locations are as follows:
  1. accumulator
  2. always contains 0
  3. character input
  4. character output
The four lines of code below demonstrate the shortest jump:
        rssb   acc       ; set acc to 0
rssb $+2 ; set acc to loop offset
rssb ip ; subtract acc from ip
rssb $-loop ; the loop offset
The code below implements hello world for the RSSB virtual computer. The sum deserves an explanation. Each character is subtracted from sum until sum passes zero, indicating all character have been printed. The final value of sum is the offset required by the conditional jump!
loop    rssb   acc       ; acc = character from ptr
ptr rssb hello

rssb out ; display character

rssb zero ; acc = -acc

rssb zero ; always skipped

rssb sum ; subtract acc from sum

rssb ip ; skipped if sum is <0
; otherwise jump to 0

one rssb acc ; subtract 1 from ptr
rssb one
rssb ptr

rssb acc ; jump to loop
rssb loopoff
rssb ip
loopoff rssb $-loop

sum rssb -1116

rssb 33 ; '!'
rssb 100 ; 'd'
rssb 108 ; 'l'
rssb 114 ; 'r'
rssb 111 ; 'o'
rssb 87 ; 'W'
rssb 32 ; ' '
rssb 44 ; ','
rssb 111 ; 'o'
rssb 108 ; 'l'
rssb 108 ; 'l'
rssb 101 ; 'e'
hello rssb 72 ; 'H'
If you can improve the code above, or you've seen any other programs inplemented with RSSB, please leave a message below.

3 comments:

  1. Here's the improved Hello World for RSSB. This code is 2 lines shorter and a few cycles faster thanks to the following improvements:

    * sum has been moved to the instruction which is always skipped

    * loopoff has been moved to an opcode which is conveniently the correct value

    * the accumulator is no longer set to 0 before loading characters

    Here's the code:

    ------------------------------
    loop rssb acc ; acc = character from ptr
    ptr rssb hello

    rssb out ; display character

    rssb zero ; acc = -acc

    sum rssb -1116 ; always skipped

    rssb sum ; subtract acc from sum

    rssb ip ; skipped if sum is < 0
    ; otherwise jump to 0

    one rssb acc ; subtract 1 from ptr
    loopoff rssb one
    rssb ptr

    rssb acc ; jump to loop
    rssb loopoff
    rssb ip

    rssb 33+6 ; '!'
    rssb 100+6 ; 'd'
    rssb 108+6 ; 'l'
    rssb 114+6 ; 'r'
    rssb 111+6 ; 'o'
    rssb 87+6 ; 'W'
    rssb 32+6 ; ' '
    rssb 44+6 ; ','
    rssb 111+6 ; 'o'
    rssb 108+6 ; 'l'
    rssb 108+6 ; 'l'
    rssb 101+6 ; 'e'
    hello rssb 72 ; 'H'
    ------------------------------

    ReplyDelete
  2. Can anyone explain why there are words before the rssb instruction in some lines?

    ReplyDelete

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