This is a contrived example to demonstrate all four jump instructions. In doing so, it violates some principles of MIPS programming. First, the example deliberately jumps back and forth through the main program, an example of "spaghetti code" or unbridled "GO TO" statements. Second, the call/return for the multiply_them routine is non-standard, avoiding the use of ra.
Given that, we see that all the jump instructions work as advertised.
mul $v0,$a0,$a1 # v0 = a0*a1 div $v0,$a0,$a1 # v0 = a0/a1
The multiply operation is done by a single instruction. The divide operation is expanded to a macro.
The example illustrates two rotate instructions:
rol $t1,$t0,$t4 # t1 = t0 rotated left by t4
ror $t2,$t0,$t4 # r2 = t0 rotated right by t4
The first instruction (rotate left) is a pseudo-instruction expanded into a macro. The second instruction (rotate right) is a machine instruction.
Download mips3.zip
# mips3.s illustrates jump instructions
.global main
.text
/* This directive tells the assembler don't optimize
* the order of the instructions and don't insert
* 'nop' instructions after jumps and branches.
*/
.set noreorder
/*********************************************************************
* main()
* This is where the PIC32 start-up code will jump to after initial
* set-up.
********************************************************************/
.ent main # directive that marks symbol 'main' as function in ELF output
main:
li $a0,24
li $a1,4
j first
nop
second:
jal divide_them
nop
j done
nop
first:
la $s2,multiply_them
jalr $s1,$s2 # This is bad practice (should use ra)
nop
move $v1,$v0
j second
nop
done:
li $t0,0xABCD1234
li $t4,8
rol $t1,$t0,$t4
ror $t2,$t0,$t4
jr $ra
nop
multiply_them:
mul $v0,$a0,$a1
jr $s1 # this is bad practice (should use ra)
nop
divide_them:
div $v0,$a0,$a1
jr $ra
nop
.end main # directive that marks end of 'main' function
# and registers size in ELF output
--- C:\pic32\test\mips3.s ----------------------------------------------------------------------
1: # mips3.s illustrates jump instructions
2:
3: .global main
4:
5: .text
6:
7: /* This directive tells the assembler don't optimize
8: * the order of the instructions and don't insert
9: * 'nop' instructions after jumps and branches.
10: */
11: .set noreorder
12:
13: /*********************************************************************
14: * main()
15: * This is where the PIC32 start-up code will jump to after initial
16: * set-up.
17: ********************************************************************/
18:
19: .ent main # directive that marks symbol 'main' as function in ELF output
20:
21: main:
9D000018 24040018 addiu a0,zero,24 22: li $a0,24
9D00001C 24050004 addiu a1,zero,4 23: li $a1,4
9D000020 0B40000E j 0x9d000038 24: j first
9D000024 00000000 nop 25: nop
26:
27: second:
9D000028 0F400020 jal 0x9d000080 28: jal divide_them
9D00002C 00000000 nop 29: nop
9D000030 0B400015 j 0x9d000054 30: j done
9D000034 00000000 nop 31: nop
32:
33: first:
9D000038 3C129D00 lui s2,0x9d00 34: la $s2,multiply_them
9D00003C 26520074 addiu s2,s2,116
9D000040 02408809 jalr s1,s2 35: jalr $s1,$s2 # This is bad practice (should use ra)
9D000044 00000000 nop 36: nop
9D000048 00401821 addu v1,v0,zero 37: move $v1,$v0
9D00004C 0B40000A j 0x9d000028 38: j second
9D000050 00000000 nop 39: nop
40: done:
9D000054 3C08ABCD lui t0,0xabcd 41: li $t0,0xABCD1234
9D000058 35081234 ori t0,t0,0x1234
9D00005C 240C0008 addiu t4,zero,8 42: li $t4,8
9D000060 000C4823 subu t1,zero,t4 43: rol $t1,$t0,$t4
9D000064 01284846 rotrv t1,t0,t1
9D000068 01885046 rotrv t2,t0,t4 44: ror $t2,$t0,$t4
45:
9D00006C 03E00008 jr ra 46: jr $ra
9D000070 00000000 nop 47: nop
48:
49: multiply_them:
9D000074 70851002 mul v0,a0,a1 50: mul $v0,$a0,$a1
9D000078 02200008 jr s1 51: jr $s1 # this is bad practice (should use ra)
9D00007C 00000000 nop 52: nop
53:
54: divide_them:
9D000080 00A001F4 teq a1,zero 55: div $v0,$a0,$a1
9D000084 0085001A div a0,a1
9D000088 2401FFFF addiu at,zero,-1
9D00008C 14A10002 bne a1,at,0x9d000098
9D000090 3C018000 lui at,0x8000
9D000094 008101B4 teq a0,at
9D000098 00001012 mflo v0
9D00009C 03E00008 jr ra 56: jr $ra
9D0000A0 00000000 nop 57: nop
Maintained by John Loomis, last updated 18 July 2008