Why does lw return “Immediate value is too large for field:” in QtSpim?

Question

I am running QtSpim 9.1.7 on 64-bit Windows 7. Under Simulator / Settings / MIPS I have all options ticked ("Bare Machine", "Accept Pseudo Instructions", "Enable Delayed Branches", "Enabled Delayed Loads", "Enable Mapped IO", "Load Exception Handler").

I am able to run basic code to add values in QtSpim if I store hard-coded values using ori as follows: ori $t0,$0,4. Now I want to load and store data from memory, but lw isn't working.

.text
.globl main
main:
lw $t0,num1
.data
num1: .word 4

When I Reinitialise and load the above file, I get the following error message: "Immediate value is too large for field:"

If I then try to step through the code, I get as far as line [00400024]. Stepping over that line, I get these error messages: "Exception occurred at PC=0x00400024" / "Bad address in data/stack read: 0x00000000"

For reference, my code is parsed as:

User Text Segment [00400000]..[00440000]
[00400000] 8fa40000  lw $4, 0($29)            ; 183: lw $a0 0($sp) # argc 
[00400004] 27a50004  addiu $5, $29, 4         ; 184: addiu $a1 $sp 4 # argv 
[00400008] 24a60004  addiu $6, $5, 4          ; 185: addiu $a2 $a1 4 # envp 
[0040000c] 00041080  sll $2, $4, 2            ; 186: sll $v0 $a0 2 
[00400010] 00c23021  addu $6, $6, $2          ; 187: addu $a2 $a2 $v0 
[00400014] 0c100009  jal 0x00400024 [main]    ; 188: jal main 
[00400018] 00000000  nop                      ; 189: nop 
[0040001c] 3402000a  ori $2, $0, 10           ; 191: li $v0 10 
[00400020] 0000000c  syscall                  ; 192: syscall # syscall 10 (exit) 
[00400024] 8c080000  lw $8, 0($0) [num1]      ; 4: lw $t0,num1

The first thing that occurs to me is that 0($0) is surely not the address of num1. Is my data declaration incorrect? If so, what have I done wrong?

I don't know if it's related, but I got syntax errors for the following la and li commands (when I was trying to find an alternative to lw):

• la $a0,num1
• li $t0,4

Answer 1

This appears to be a bug in QtSpim. The same code works in two other environments (with slightly different versions of QtSpim, so I'm not sure if it's a regression in 9.1.7 or the environment)...

Environment 1: EE380 CGI SPIM

Accessible at http://cgi.aggregate.org/cgi-bin/cgispim.cgi and running SPIM 6.3a. It runs the code without returning an error and generates the following execution trace:

SPIM Version 6.3a of January 14, 2001
Copyright 1990-2000 by James R. Larus (larus@cs.wisc.edu).
All Rights Reserved.

[0x00400000]    0x8fa40000  lw $4, 0($29)                   ; 102: lw $a0, 0($sp) # argc
[0x00400004]    0x27a50004  addiu $5, $29, 4                ; 103: addiu $a1, $sp, 4 # argv
[0x00400008]    0x24a60004  addiu $6, $5, 4                 ; 104: addiu $a2, $a1, 4 # envp
[0x0040000c]    0x00041080  sll $2, $4, 2                   ; 105: sll $v0, $a0, 2 
[0x00400010]    0x00c23021  addu $6, $6, $2                 ; 106: addu $a2, $a2, $v0 
[0x00400014]    0x0c100008  jal 0x00400020 [main]           ; 107: jal main 
[0x00400020]    0x3c011001  lui $1, 4097 [num1]             ; 4: lw $t0,num1
[0x00400024]    0x8c280000  lw $8, 0($1) [num1]

Environment 2: Ubuntu

The same code executes without error on QtSpim 9.1.6 on 32-bit Ubuntu 11.10 and generates the following:

User Text Segment [00400000]..[00440000]
[00400000] 8fa40000  lw $4, 0($29)            ; 183: lw $a0 0($sp) # argc 
[00400004] 27a50004  addiu $5, $29, 4         ; 184: addiu $a1 $sp 4 # argv 
[00400008] 24a60004  addiu $6, $5, 4          ; 185: addiu $a2 $a1 4 # envp 
[0040000c] 00041080  sll $2, $4, 2            ; 186: sll $v0 $a0 2 
[00400010] 00c23021  addu $6, $6, $2          ; 187: addu $a2 $a2 $v0 
[00400014] 0c100009  jal 0x00400024 [main]    ; 188: jal main 
[00400018] 00000000  nop                      ; 189: nop 
[0040001c] 3402000a  ori $2, $0, 10           ; 191: li $v0 10 
[00400020] 0000000c  syscall                  ; 192: syscall # syscall 10 (exit) 
[00400024] 3c011001  lui $1, 4097 [num1]      ; 4: lw $t0,num1 
[00400028] 8c280000  lw $8, 0($1) [num1]      

Answer 2

This is not a bug. The simulator is running in bare mode, simulating a bare MIPS machine. The load instruction only has a 16-bit field in this mode, so it really cannot hold the address. In the non-bare mode, spim will generate a two-instruction sequence (lui, lw) to properly address the datum.

Answer 3

.data should comes before everything

as follow :

.data
num1: .word 4

.text
.globl main
main:
lw $t0,num1

li $v0, 10
syscall
.end