MIPS 32 bit, assembler: push and pop

Question

I'm having trouble with my code, MARS will not assemble it, because I use a wrong type of register for the sw instruction.

Why cant I use this line

sw $so, 0($sp)

Is there something wrong with using s type register with sw? I want to save the value of $so on stack, how should I go about it?

full code:

.data   
.text
f1:
    li $s0, 30
    addi $sp, $sp, -4
    sw $so, 0($sp)
    jal f2
    lw $v2, 0($v1)
    add $sp, $zero, $zero
    addi $sp, $sp, -4
    add $sp, $zero, $zero
    j Exit
f2:
    addi $sp, $sp, -4
    sw $sp, 0($ra)
    jal f3
    lw $to, 0($vo)
    srl $t0, $to, 1
    add $to, $to, $to
    lw $ra, 0($sp)
    addi $sp, $sp, 4
    jr $ra
f3:
    addi $vo, $so, 1
    jr $ra
Exit: 
    result .asciiz "Value is "
    la $v2, result 
    syscall

Edit:

thanks for the answers, I got things more correct, and now I have a problem at line 16 in my new code. What should happen before I save an address on stack, so that the following error won't appear? Error:

line 16: Runtime exception at 0x00400028: Cannot write directly to text segment!0x00400010

code:

.data
    result: .asciiz "Value is "
.text
f1:
    li $a0, 30          #n=30
    addi $sp, $sp, -4       #set sp back for save
    sw $a0, 0($sp)          #save a0 to stack
    jal f2              
    lw $a1, 0($v1)          
    add $sp, $zero, $zero
    addi $sp, $sp, -4
    add $sp, $zero, $zero
    j Exit
f2:
    addi $sp, $sp, -4
    sw $sp, 0($ra)
    jal f3
    lw $t0, 0($v0)          #
    srl $t0, $t0, 1
    add $t0, $t0, $t0
    lw $ra, 0($sp)
    addi $sp, $sp, 4
    jr $ra
f3:
    addi $v0, $a0, 1        #
    jr $ra
Exit: 
    li $v0, 4
    la $a2, result 
    syscall

Answer 1

sw $s0, 0($sp)

should write $S0 on top of your stack