Emu, A MIPS Emulator

Introduction

Emu is a MIPS emulator. In this project, we complete the print_instruction and execute_instruction functions.

Setup and Installation

To run Emu, use the following command:

$ make dcc emu.c ram.c registers.c execute_instruction.c print_instruction.c -o emu
$ ./emu ...

Functions

print_instruction

This function is contained in print_instruction.c. It is given a MIPS instruction as a 32-bit unsigned integer value, and needs to print out the assembler for the instruction.

For example, print_instruction(0x00851820) should print

add $3, $4, $5

emu takes several command-line arguments. The -p option indicates the rest of the command is hexadecimal integers describing instructions that print_instruction will be called on —, for example:

$ make dcc emu.c ram.c registers.c execute_instruction.c print_instruction.c -o emu
$ ./emu -p 0x00851820 
[00400024] 00851820 add $3, $4, $5

The assembler instruction should be quoted since the $ character has special meaning in Shell.

emu also has a -P option, which takes a file of assembler statements, converts them to integers, and calls print_instruction for each integer, for example:

$ ./emu -P print10.s 
[00400024] 34080001 ori $8, $0, 1 
[00400028] 2901000B slti $1, $8, 11 
[0040002C] 10200009 beq $1, $0, 9 
[00400030] 00082020 add $4, $0, $8 
[00400034] 34020001 ori $2, $0, 1 
[00400038] 0000000C syscall 
[0040003C] 3404000A ori $4, $0, 10 
[00400040] 3402000B ori $2, $0, 11 
[00400044] 0000000C syscall 
[00400048] 21080001 addi $8, $8, 1 
[0040004C] 0401FFF7 bgez $0, -9 
[00400050] 03E00008 jr $31

execute_instruction

This function is contained in execute_instruction.c. It is given a MIPS instruction as a 32-bit unsigned integer value

You implement instructions by appropriately calling the functions get_register, set_register, get_byte and set_byte. execute_instruction must also update the program counter.

emu has a -e option, will also accept assembler statements, convert them to integers, and calls execute_instruction for each integer, then print the value of registers. For example:

$ ./emu -e ‘add $4, $14, $12’ 
R0 [$zero] = 00000000 
R1 [$at] = 00000000 
R2 [$v0] = 00000000 
R3 [$v1] = 00000000 
R4 [$a0] = 0000000A 
R5 [$a1] = 00000000 
R6 [$a2] = 00000000 
R7 [$a3] = 00000000 
R8 [$t0] = 00000000 
R9 [$t1] = 00000001 
R10 [$t2] = 00000002 
R11 [$t3] = 00000003 
R12 [$t4] = 00000004 
R13 [$t5] = 00000005 
R14 [$t6] = 00000006 
R15 [$t7] = 00000007 
R16 [$s0] = 00000000 
R17 [$s1] = 00000000 
R18 [$s2] = 00000000 
R19 [$s3] = 00000000 
R20 [$s4] = 00000000
R21 [$s5] = 00000000 
R22 [$s6] = 00000000 
R23 [$s7] = 00000000 
R24 [$t8] = 00000000 
R25 [$t9] = 00000000 
R26 [$k0] = 00000000 
R27 [$k1] = 00000000 
R28 [$gp] = 10008000 
R29 [$sp] = 7FFFF8E4 
R30 [$fp] = 00000000 
R31 [$ra] = 00400018

emu also has a -E option, which like -P takes a file of assembler statements, converts them to integers, and calls execute_instruction multiple times to execute them. For example:

$ ./emu -E sum_100_squares.s 338350

emu can also be run interactively:

$ ./emu add_memory.s 
PC = [00400024] 34080011 ori $8, $0, 17 emu > h 

In interactive mode, available commands are:

• s step (execute one instruction)
• r execute all remaining instructions
• q quit
• h this help message
• P print Program
• R print Registers
• D print Data segment
• S print Stack segment
• T print Text segment

Entering nothing will re-send the previous command.

emu > R 
R0 [$zero] = 00000000 
R1 [$at] = 00000000 
R2 [$v0] = 00000000 
R3 [$v1] = 00000000 
R4 [$a0] = 00000000 
R5 [$a1] = 00000000 
R6 [$a2] = 00000000 
R7 [$a3] = 00000000 
R8 [$t0] = 00000000 
R9 [$t1] = 00000001 
R10 [$t2] = 00000002 
R11 [$t3] = 00000003 
R12 [$t4] = 00000004 
R13 [$t5] = 00000005 
R14 [$t6] = 00000006 
R15 [$t7] = 00000007 
R16 [$s0] = 00000000 
R17 [$s1] = 00000000 
R18 [$s2] = 00000000 
R19 [$s3] = 00000000 
R20 [$s4] = 00000000 
R21 [$s5] = 00000000 
R22 [$s6] = 00000000 
R23 [$s7] = 00000000 
R24 [$t8] = 00000000 
R25 [$t9] = 00000000 
R26 [$k0] = 00000000 
R27 [$k1] = 00000000 
R28 [$gp] = 10008000 
R29 [$sp] = 7FFFF8E4 
R30 [$fp] = 00000000 
R31 [$ra] = 00400018 
PC = [00400024] 34080011 ori $8, $0, 17 emu > s 
PC = [00400028] 3C011001 lui $1, 4097 emu > s 
PC = [0040002C] AC280000 sw $8, 0($1) emu > D 
[10000000..1000FFFC] 00000000 [10010000] 00000011 [10010004] 00000019 [10010008] 0000002A 

MIPS Architecture

You only need to implement the following subset of instructions and system calls; emu will only be tested on these:

Assembler Description C Bit Pattern

• add $d, $s, $t
• add d = s + t 000000ssssstttttddddd00000100000
• sub $d, $s, $t
• subtract d = s – t 000000ssssstttttddddd00000100010
• mul $d, $s, $t
• multiply to low d = s * t 011100ssssstttttddddd00000000010
• and $d, $s, $t
• and d = s & t 000000ssssstttttddddd00000100100
• or $d, $s, $t
• or d = s l t 000000ssssstttttddddd00000100101
• xor $d, $s, $t
• xor d = s ^ t 000000ssssstttttddddd00000100110
• sllv $d, $t, $s
• shift left d = t << s 000000ssssstttttddddd00000000100
• srlv $d, $t, $s
• shift right d = t >> s 000000ssssstttttddddd00000000110
• slt $d, $s, $t
• set on less than d = (s < t) 000000ssssstttttddddd00000101010
• addi $t, $s, I
• add immediate t = s + I 001000ssssstttttIIIIIIIIIIIIIIII
• andi $t, $s, I
• and with immediate t = s & I 001100ssssstttttIIIIIIIIIIIIIIII
• ori $t, $s, I
• or with immediate t = s l I 001101ssssstttttIIIIIIIIIIIIIIII
• xori $t, $s, I
• xor with immediate t = s ^ I 001110ssssstttttIIIIIIIIIIIIIIII
• sll $d, $t, I
• shift left immediate d = t << I 00000000000tttttdddddIIIII000000
• srl $d, $t, I
• shift right immediate d = t >> I 00000000000tttttdddddIIIII000010
• slti $t, $s, I
• set on less than immediate t = (s < I) 001010ssssstttttIIIIIIIIIIIIIIII
• lui $t, I
• load upper immediate t = I << 16 00111100000tttttIIIIIIIIIIIIIIII
• lb $t, O($b)
• load byte t = *(int8*)(b + O) 100000bbbbbtttttOOOOOOOOOOOOOOOO
• lh $t, O($b)
• load half word t = *(int16*)(b + O) 100001bbbbbtttttOOOOOOOOOOOOOOOO
• lw $t, O($b)
• load word t = *(int32*)(b + O) 100011bbbbbtttttOOOOOOOOOOOOOOOO
• sb $t, O($b)
• store byte *(uint8*)(b + O) = (t & 0xff) 101000bbbbbtttttOOOOOOOOOOOOOOOO
• sh $t, O($b)
• store half *(uint16*)(b + O) = (t & 0xffff) 101001bbbbbtttttOOOOOOOOOOOOOOOO
• sw $t, O($b)
• store word *(uint32*)(b + O) = t 101011bbbbbtttttOOOOOOOOOOOOOOOO
• beq $s, $t, I
• branch on equal if (s == t) PC += I<<2; else PC += 4; 000100ssssstttttIIIIIIIIIIIIIIII
• bne $s, $t, I
• branch on not equal if (s != t) PC += I<<2; else PC += 4; 000101ssssstttttIIIIIIIIIIIIIIII
• blez $s, I
• branch less than or equal than zero if (s <= 0) PC += I<<2; else PC += 4; 000110sssss00000IIIIIIIIIIIIIIII
• bgtz $s, I
• branch greater than zero if (s > 0) PC += I<<2; else PC += 4; 000111sssss00000IIIIIIIIIIIIIIII
• bltz $s, I
• branch on less than zero if (s < 0) PC += I<<2; else PC += 4; 000001sssss00000IIIIIIIIIIIIIIII
• bgez $s, I
• branch on greater than or equal to zero if (s >= 0) PC += I<<2; else PC += 4; 000001sssss00001IIIIIIIIIIIIIIII
• j X
• jump PC = (PC & 0xF0000000) | (X << 2) 000010XXXXXXXXXXXXXXXXXXXXXXXXXX
• jal X
• jump and link $ra = PC + 4; PC = (PC & 0xF0000000) | (X << 2) 000011XXXXXXXXXXXXXXXXXXXXXXXXXX
• jr $s
• jump register PC = s 000000sssss000000000000000001000
• syscall
• system call determined by $v0 00000000000000000000000000001100

The instruction 'Bit Pattern' uniquely identifies each instruction:

• 0: Literal bit zero
• 1: Literal bit one X: Immediate, print as hex with '0x' prefix I: Immediate, print as dec b: Base register field (number is the N in $N)
• O: Offset immediate, print as dec [lowercase letter]:
• Register field (number is the N in $N)

Arithmetic instruction should assume registers contain a signed 32-bit number. Arithmetical instruction should not attempt to stop overflows as a result of their operation.

Every instruction has a PC += 4 after the operation, except for instruction that directly change the program counter, such as branches or jumps.

You only need to implement this subset of system calls.