Rand.s

.global _start

/* PseudoRandom number Generator

    This pseudo random number generator takes the 4th byte from the linux gettime(2) syscall
    which is the time in microseconds from epotch. This will then load the number into
    r1, which then subtracts from the number until it's an integer less than 10 but greater than 0.
    This subtraction is my poor mans attempt at some kind of modulus or floor operator without getting
    overly complicated as I only need to print 1 digit.

    The result of the "fakemod" will then be fed into into an algorithm to convert
    the integer as an ascii string for output.
    The period doesn't print at the end, but everything works.

    Was constrained in terms of not being allowed to use urandom, use of urandom would not have been marked by the professor.
*/

_start:

MOV r5, #0
Numberloop:
    ADD r5, r5, #1      @r5 is my counter

    PUSH {r5}
    BL timeofday        @grab our random number, return in r1
    BL fakemod          @perform a (fake) mod operation, return in r1

    BL _numtoString

    LDR r1, =numstr
    BL _sPrint

    LDR r1, =comma
    BL _sPrint

    POP {r5}
    CMP r5, #9
    BNE Numberloop
    BEQ lasttask

lasttask:

    BL timeofday        @grab our random number, return in r1
    BL fakemod          @perform a (fake) mod operation, return in r1
    BL _numtoString

    LDR r1, =numstr
    BL _sPrint

@ This is here for testing purposes, originally _sPrint was a seperate library file in .o provided by the professor
@ I had to disassemble the object file to extract _sPrint, _strLen, and findEnd so to avoid having to give out
@ the object file. Nothing about the code behaviour has changed. A period is still not printed at the end.

@This was to see if maybe the _sPrint function being used in the object file might have been the reason for the missing period.
@As at the time I did not have it disassembled

    MOV r7, #4              @set the write bit (4) in register 7 to write to console
    MOV r0, #1              @set WRITE destination to STDOUT (terminal)
    LDR r1, =period         @Loads data store at the address ID'd by the label, into r1 for output
    MOV r2, #2              @Set R2 to be the max size output prompt.
    SWI 0                   @RUN/EXECUTE WRITE syscall

@missing newline printing, commented it out for testing.
    @LDR r1 =newline
    @BL _sPrint

    B _exit


@functions here

timeofday:

    MOV r7, #78         @gettimeofday returns time
    LDR r0, =time       @address of holder for time_t
    MOV r1, #0          @timezone not needed
    SWI #0

    LDR r0, =time       @set r0 back to label addr
    LDRB r1, [r0, #4]   @load epotch value from r0 into r1

    MOV PC, LR          @back to calling location

fakemod:
    CMP r1, #10         @Ensuring we don't create a negative
    SUBGE r1, r1, #10   @decrease r1 by 10 until it's a single digit
    CMP r1, #10
    BGE fakemod         @if the number in r1 is still greater than 10 loop

    MOV PC, LR          @once it's an integer less than 10, go back to calling enviroment


_numtoString:

    ADD r1, r1, #48     @coverts an integer to an ascii value
    LDR r4, =numstr     @loads the numstr pointer address to r4
    STR r1, [r4]        @stores the Ascii integer representation into the numstr address

    MOV PC, LR          @so back to the BL that called this function

_sPrint:
    mov r0, #1          @sets r0 to 1 for printing
    push {r0, r1, lr}   @saves r0,r1 and lr before branching to _strlen
    bl _strLen
    pop {r0, r1, lr}    @restores the saved register
    mov r7, #4          @sets output to console
    SWI 0               @execute syscall
    mov pc, lr          @back to call enviroment


_strLen:
  mov   r2, #0          @r2 is the counter

findEnd:
 ldrb   r0, [r1], #1    @load a value from r1 into r0 and increment r1 address offset
  add   r2, r2, #1      @increment counter
  cmp   r0, #0          @compare to 0, end of string
  bne findEnd           @if not 0, loopback
  sub   r2, r2, #1      @subtract 1 so we're not one step ahead of desire pointer location
  mov   pc, lr




_exit:
    MOV r7, #1
    SWI #0

.data
time:       .space 8
comma:      .asciz ", "
newline:    .asciz  "\n"
numstr:     .asciz " "
period:     .ascii ".\n"