initial stack pointer is not 0x7fffffff in MIPS

Question

I've assembled this a short MIPS assembly code using QtSPIM simulator. I am appending the code for completion:

.text
.globl main
main:
 subu $sp,$sp,32 # Stack frame is 32 bytes long
 sw $ra,20($sp) # Save return address
 sw $fp,16($sp) # Save old frame pointer
 addiu $fp,$sp,28 # Set up frame pointer
 li $a0,10 # Put argument (10) in $a0
 jal fact # Call factorial function
 la $a0,$LC # Put format string in $a0
 move $a1,$v0 # Move fact result to $a1
 lw $ra,20($sp) # Restore return address
 lw $fp,16($sp) # Restore frame pointer
 addiu $sp,$sp,32 # Pop stack frame
 jr $ra # Return to caller
 
 .rdata
$LC:
.ascii "The factorial of 10 is %d\n\000"
 
 .text
fact:
 subu $sp,$sp,32 # Stack frame is 32 bytes long
 sw $ra,20($sp) # Save return address
 sw $fp,16($sp) # Save frame pointer
 addiu $fp,$sp,28 # Set up frame pointer
 sw $a0,0($fp) # Save argument (n)
 lw $v0,0($fp) # Load n
 bgtz $v0,$L2 # Branch if n > 0
 li $v0,1 # Return 1
 jr $L1 # Jump to code to return

$L2:
  lw $v1,0($fp) # Load n
  subu $v0,$v1,1 # Compute n - 1
  move $a0,$v0 # Move value to $a0
  jal fact # Call factorial function
  lw $v1,0($fp) # Load n
  mul $v0,$v0,$v1 # Compute fact(n-1) * n
  
$L1: # Result is in $v0
  lw $ra, 20($sp) # Restore $ra
  lw $fp, 16($sp) # Restore $fp
  addiu $sp, $sp, 32 # Pop stack
  jr $ra # Return to caller

The book I am reading on MIPS, says that the initial stack pointer when the program is run should point to location 0x7FFFFFFF in memory. Stack pointer ($sp) in the MIPS simulator is first access at instruction number 2. $sp points to an address 0x7ffffe10 as indicated by the $a1 register (0x7ffffe10 = 7ffffe14 - 4 ). How come the stack pointer address is 0x7ffffe10 and not 0x7FFFFFFF? What code changed it?

Answer 1

First, 0x7FFFFFFF is not a reasonable address for a MIPS stack pointer, because it is odd.  A MIPS stack pointer points to words, so should be an even multiple of 4.

Second, QtSPIM sets up the simulation's initial stack in a similar manner to a UNIX process — it puts command line parameters and environment variables on the stack.

Suggest you take a look at the stack in the data section and you'll most likely see strings of the environment.  (Click on the Data tab, before running the first instruction of a simulation, then view the User Stack memory area.)

Running QtSPIM on windows, for example, I see the same strings in that I see when doing set command from the command line shell cmd (cmd.exe).

If you add "Command-line arguments to pass to program" using "Run Parameters" menu item (from QtSPIM menu: "Simulator"), any strings you type there will also appear on the stack in front of the environment strings.  That will also change the initial stack pointer value used by the simulation.