Spim as Intermediate code

• We are only going to use temporary registers $t0 - $t7
• All data is going to be stored in word size (32 bit) locations.
• Data manipulations are always done on registers.
• Comments start with # and go to the end of the line.
• Data and code are marked using the .data and .text directives, respectively. Be sure that in your generated code, the data labels appear in a .data section and that intermediate code and labels for control flow appear in a .text section.

        .data
  item: .word 1           # allocate a word of memory, initial value 1
  arr:  .word 0,0,0,0,0   # allocate 5 words of memory, initial value 0
        .text
  main:  lw $t0,item
         . . .
  

  The above code allocates a word for variable item with initial value 1 and 5 words for arr.

Addressing Modes:
Spim allows various addressing modes, a subset of which we can use during intermediate code generation

Format	Address Computation
register	indicates the register
(register)	contents of the register
imm	immediate value
imm(register)	immediate value + contents of register
symbol	address of symbol

Instructions

1. Data Movement
   • lw temp, var -- copies memory value stored at var into register temp
   • li temp, number -- copies given number into register temp
   • la temp, var -- loads the address of var into register temp
   • sw temp, var -- copies temp register contents into a memory location var
   • move temp1,temp2 -- copy the contests of temp2 to temp1.
2. Arithmetic
   • add temp1, temp2, temp3 -- added the contents of registers temp2 and temp3, storing the result in temp1.
   • sub temp1, temp2, temp3 -- subtracts the contents of register temp3 from temp2, storing the result in temp1.
   • mul temp1, temp2, temp3 -- multiplies the contents of registers temp2 and temp3, storing the result in temp1.
   • div temp1, temp2, temp3 -- divides the contents of register temp3 into temp2, storing the result in temp1.
   • neg temp1, temp2 -- negate the contents of register temp2, storing the result in temp1.
3. Comparison
   • seq temp1, temp2, temp3 -- Set register temp1 to 1 if contents of register temp2 equals contents of register temp3, 0 otherwise
   • sge temp1, temp2, temp3 -- Set register temp1 to 1 if contents of register temp2 is greater than or equal contents of register temp3, 0 otherwise
   • sgt temp1, temp2, temp3 -- Set register temp1 to 1 if contents of register temp2 is greater than contents of register temp3, 0 otherwise
   • sle temp1, temp2, temp3 -- Set register temp1 to 1 if contents of register temp2 is less than or equal contents of register temp3, 0 otherwise
   • slt temp1, temp2, temp3 -- Set register temp1 to 1 if contents of register temp2 is less than contents of register temp3, 0 otherwise
   • sne temp1, temp2, temp3 -- Set register temp1 to 1 if contents of register temp2 is not equal contents of register temp3, 0 otherwise
4. Branching
   • b label -- unconditionally branch to the instruction at the label.
   • beqz temp, label -- branch to the instruction at the label if the contest of temp equals 0.
   • bnez temp, label -- branch to the instruction at the label if the contest of temp does not equal 0.
   • jal label -- unconditionally jump to the instruction at label. Save the address of the instruction following the jump in register 31 ($ra).
   • jr temp -- unconditionally jump to the instruction at the address stored in register temp
5. System Services (i/o). The following shows the instructions required to read and print integers, as well as designate the termination of the program.
   • Printing integers:

     li $v0,1     # system code for printing integers placed into v0 
     li $a0,5     # load the integer into register a0. This instruction 
                  # could also be a move or lw into a0.
     syscall      # print it
     

   • Reading integers:

     li $v0,5     # system code for reading integers placed into v0 
     syscall      # read it
     sw $v0,x     # move the integer into new location. This instruction 
                  # could also be a move from v0 into a register.
     

Using the Spim Interpreter

The interface is fairly self-explanatory, however you will probably only need to use a few of the buttons.

• quit
• load -- provide a file name to load into the simulator. If there are syntax problems with the file, that information appears in the bottom window. As you might expect, your spim code will not run with syntax errors.
• run
• clear -- If you need to load or reload a new file into the simulator, it is a good idea to clear out the memory and registers to be sure nothing remains of the old program to cause problems. If you want to run a loaded program again, use clear to clear out the registers so that old information is not used.