Variables in memory and constants

This program uses three adjacent four-byte slots in memory: price and shipping start with values, and total holds the result. It loads the two inputs into registers, adds them and a named constant, then stores the sum back in memory.

.eqv TAX 20

.data
price:    .word 250
shipping: .word 35
total:    .space 4

.text
main:
    la $t0, price       # address of the first word
    lw $t1, 0($t0)      # load price
    lw $t2, 4($t0)      # load shipping
    add $t1, $t1, $t2   # price + shipping
    addi $t1, $t1, TAX  # add the constant
    sw $t1, 8($t0)      # store the result in total

    li $v0, 10          # exit
    syscall

Follow the values through the program. la puts the address of price in $t0. The two lw instructions load 250 into $t1 and 35 into $t2; add and addi leave 305 in $t1. Finally, sw stores that value at total. The arithmetic happens in registers, while the inputs and result live in memory.

The three data labels mark consecutive four-byte slots. From the address in $t0, 0($t0) reaches price, 4($t0) reaches shipping, and 8($t0) reaches total. These offsets count bytes. Here price and shipping are initialized with .word; total is four bytes reserved with .space 4. The Playground loader happens to show newly reserved bytes as zero; .space itself only reserves them. .eqv TAX 20 gives the assembler a name for the number 20 and uses no memory. These lines beginning with a dot tell the assembler how to prepare the program before it runs.

In the Playground's default layout, price begins at 0x10010000. Type 10010000 in the memory panel's address box and compare the three slots before and after running:

Label	Address	Playground before run	After run
price	0x10010000	FA 00 00 00	unchanged
shipping	0x10010004	23 00 00 00	unchanged
total	0x10010008	00 00 00 00	31 01 00 00

The final value, 305, is 0x00000131 in hexadecimal. In the Playground's little-endian memory, its least significant byte goes at the lowest address, so the byte view shows 31 01 00 00. Select W to group those bytes into a word and see the value in the usual order.

Now try a change that makes the offsets matter. Select Open in editor on the program above. Add discount: .word 15 between shipping and total. Then insert these instructions immediately after lw $t2, 4($t0):

    lw $t3, 8($t0)      # load discount
    sub $t1, $t1, $t3   # subtract discount from price

The new slot takes offset 8 and moves total to offset 12, so change the final store to sw $t1, 12($t0). Before running, predict the modified program's final $t1 and the four bytes at total. Then run it and check the register and memory panels. $t1 should hold 290, and total should show 22 01 00 00 in the byte view (0x00000122).