Chapter 3: Instructions

Chapter 3: Instructions CS 447 Jason D. Bakos

Assembly Language Instructions • We’re going to use the MIPS R2000 instruction set for the projects in this course • Review: Assembly language instructions are English words that represent binary machine language instructions

Arithmetic/Logical Instructions • Arithmetic instructions are composed of an operation and three registers • <op> <destreg> <operand1> <operand2> • There are 32 32-bit general-purpose integer registers that we can use (R0 is hardwired to value 0 for A/L operands) • Ex: add $1, $2, $3 • When writing assembly language programs, comments can be added after any instruction by using the ‘#’ character • Some architectures only use 2 registers • The result is stored in the first operand register • Some of our instructions will use this convention • Appendix A in the textbook lists all

Immediate Arithmetic/Logical Instructions • Most arithmetic/logical instructions have an “immediate” version • Immediate instructions have an ‘i’ at the end of the instruction name • Immediate instructions allow us to specify an immediate/constant/literal in lieu of a register operand • Ex: add $1, $2, 16 • We only have 16-bits for the immediate, so we can only represent 216 unique values • Use decimal numbers for the immediate value in the assembly-language instruction

A/R Instructions • MIPS R2000 A/L-instructions (complete) • add, addu, addi, addiu • and, andi • div, divu (two registers) • mult, multu (two registers) • nor • or, ori • sll, sllv, sra, srav, srl, srlv • sub, subu • xor, xori

Pseudoinstructions • Pseudoinstructions are assembled into more than 1 machine instruction • There’s no real machine instruction for them • Ex. abs rdest, rsrc • Assembles into three instructions • move the contents of rsrc into rdest • if rdest is greater than 0, skip over the next instruction • We haven’t covered conditional branches yet • subtract rdest from 0 • Appendix A specifies which are pseudoinstructions

Design Considerations • Making most arithmetic instructions have 3 operands simplifies the hardware • This is a design decision • Having a variable number of operands would greatly complicate the hardware • Limiting ourselves to 32 registers is also a design decision to keep things on the processor die smaller • Influences clock cycle time • Gives us more real estate for other things on our core • Less registers, less decoding logic, etc.

Registers • MIPS assembly language also has symbolic names for the registers • $0 => constant 0 • $1 => $at (reserved for assembler) • $2,$3 => $v0,$v1 (expression evaluation and results of a function) • $4-$7 => $a0-$a3 (arguments 1-4) • $8-$15 => $t0-$t7 (temporary values) • Used when evaluating expressions that contain more than two operands (partial solutions) • Not preserved across function calls • $16-$23 => $s0->$s7 (for local variables, preserved across function calls) • $24, $25 => $t8, $t9 (more temps) • $26,$27 => $k0, $k1 (reserved for OS kernel) • $28 => $gp (pointer to global area) • $29 => $sp (stack pointer) • $30 => $fp (frame pointer) • $31 => $ra (return address, for branch-and-links)

Memory and Load/Store Instructions • As we’ve already seen, a processor can only use data stored in its registers as operands and targets for computation • Typically, before computation can be performed, data must be loaded from main memory • After the computation is complete, the results must be stored back to main memory

Memory • Computer memory is byte-addressed but the MIPS architecture requires alignment of loads and stores on the 32-bit (4 byte/word) boundary • This means that all memory references must be an even multiple of 4 • When addressing 4-byte words, you need to provide the address of the lowest-addressed byte in the word

Load/Store Instructions • Most commonly used: • lw, sw, lh, sh, lb, sb, la • Use: • lw <rdest>, address • address can be represented as: • symbolic address (that you declare in SPIM) • symbolic address with register index • decimal_offset($basereg) • This is the way that the instruction is translated • loads/stores from contents of $basereg+decimal_offset

Load/Store Instructions • SPIM treats loads and stores with symbolic addresses as pseudoinstructions • loads the address of the data segment as an immediate into $1 • lui $1, 4097 • data starts at hex address 10010000 • If an index register is provided, adds the contents of the index reg to $1 • addu $1, $1, rindex • loads 0($1) into rdest • lw rdest, offset_of_variable($1) • This is an example of base-displacement addressing • This is why R1 is reserved when you write your programs

Load/Store Instructions • When loading or storing halfwords and bytes, the address points to a aligned word • The halfword would be the LOWER (least significant) halfword of the word that the address is pointing to • The byte world be the LOWEST (least significant) byte of the word being that the address is pointing to

Constant-Manipulating Instructions • lui rt, imm • Load the immediate imm into the upper halfword of register rt • The lower bits are set to 0 • li rt, imm • Load the imm into the lower halfword of register rt • The upper bits are set to 0

Encoding Instructions • The MIPS architecture has a 32-bit binary instruction format for each class of instruction • A/L, constant manip, comparison, branch, jump, load/store, data movement, floating-point • Even though we’ve only covered 3 of these classes, let’s look at how we encode the instructions we’ve learned…

Encoding/Translating A/L Instructions • First (most significant, left-most) 6-bits of ALL instructions is the op-code • The opcode defines the instruction • Doesn’t this only give us 26=64 different instructions? • Yes! But all A/L instructions, excluding the ones which have immediates, has an opcode of 0 but has a 6-bit field at the end of the instruction that specifies the operation

Encoding/Translating A/L Instructions • Non-immediate A/L instructions: • 6 bits – opcode (usually 0) • 5 bits – rs (operand 1) • 5 bits – rt (operand 2) • 5 bits – rd (destination) • 5 bits – 0’s or shift amount • 6 bits – operation code

Encoding/Translating A/L Instructions • Immediate A/L instructions • 6 bits – opcode (now unique) • 5 bits – rs (operand 1) • 5 bits – rt (destination) • 16 bits – immediate (understood to be 0-extended)

Encoding/Translating Load/Store Instructions • 6 bits – opcode • 5 bits – rs (base register) • 5 bits – rt (load from/store to) • Sometimes the whole register isn’t used • Half words and bytes • 16 bits – offset

Encoding/Translating Constant-Manip. Instructions • 6 bits – opcode • 5 bits – 0’s • 5 bits – rt dest. register • Half of this register will be assigned 0’s • 16 bits – immediate

Examples • Let’s compile the following C++ code segment into assembly code: • z=(a*b)+(c/d)-(e+f*g); • lw $s0,a lw $s1,b mult $s0,$s1 mflo $t0 # new instruction lw $s0,c lw $s1,d div $s0,$s1 mflo $t1 add $t0,$t0,$t1 lw $s0,e lw $s1,f lw $s2,g mult $s1,$s2 mflo $t1 add $t1,$s0,$t1 sub $t0,$t0,$t1 sw $t0,z

Examples • Note: There were many ways we could have compiled the C++ statement • Now let’s assemble the first three instructions to machine code • Assume the variables are all stored as words and are stored sequencially, starting with a, from the start of the data segment…

Examples • lw $s0,a • We know s0 is $16 • There are a few different ways we can do this, too… • First, we need to convert this instruction to a small series of instructions… • lui $1,4097 # 4097 is 1001 in hex lw 16,0($1) # 0 offset because a is first var … 00111100000000010001000000000001 => 3C011001 10001100000100000000000000000000 => 8C100000

Examples • mult $s0, $s1 • 00000010000100010000000000011000 => 02110018

Branch/Jump Instructions • Branch and jump instructions are needed for program control • if-statements (conditionals) • loops • procedure calls • Most common: • b <label> -- unconditional branch • label is a memory address (we can be symbolic ones, of course) • beq, bgez, bgezal, bgtz, blez, etc. • Conditional branches, where we’re comparing a register to 0 or two registers to determine if we’re going to branch or not • Also have al (and-link) variants, that link a return address (instruction following branch) into $31 so we can return • Branch targets are 16-bit immediate offset • offset in words… it is shifted to the left 2 bits to get byte length

Branch/Jump Instructions • j, jal • Unconditionally branches to a 26-bit pseudodirect address (not offset) • jalr, jr • Unconditionally branches to an address stored in a register

Encoding/Translating Branch/Jump Instructions • See Appendix A • These are very similar to the encoding we covered before

Data Movement and Comparison Instructions and Their Encoding • I’m going to leave the study of these instructions (from Appendix A) up to you as an exercise!

Examples • if ((a>b)&&(c=d)) e=0; else e=f; lw $s0,a lw $s1,b bgt $s0,$s1,next0 b nope next0: lw $s0,c lw $s1,d beq $s0,$s1,yup nope: lw $s0,f sw $s0,e b out yup: xor $s0,$s0,$s0 sw $s0,e out: …

Examples • Again, there are many other ways we could have tackled this… • You will eventually come up with your own strategies for “compiling” the types of constructs you want to perform…

Examples • How about a for loop? • for (i=0;i<a;i++) b[i]=i; lw $s0,a xor $s1,$s1,$s1 loop0: blt $s1,$s0,loop1 b out sll $s2,S1,2 loop1: sw $s1,b($s2) addi $s1,$s1,1 b loop0 out: …

Examples • How about a pretest while loop? • while (a<b) { a++; } lw $s0,a lw $s1,b loop0: blt $s0,$s1,loop1 b out loop1: addi $s0,Ss0,1 sw $s0,a b loop0 out: …

Examples • How about a posttest while loop? • do { a++; } while (a<b); lw $s0,a lw $s1,b loop0: addi $s0,$s0,1 sw $s0,a blt $s0,$s1,loop0 …

Procedure Calls • How about a procedure call? • The full-blown C/Pascal recursive procedure call convention is beyond the scope of this class, so we’ll skip it for now… • This is described in detail in Appendix A • You can arbitrarily implement your own procedure call convention…

Procedure Calls • Say we want to write a procedure that computes the factorial of a number, like this: int factorial (int val) { int temp=val; while (val) temp*=(--val); return temp; }

Procedure Calls • Let’s adopt the following simple convention: • We’ll pass the arguments through $a0-$a3 • Only allows for up to 4 arguments • We’ll set our return value in $v0 • The return address will be stored in $ra • This limits us to 1-level of procedure calls • We’ll assume the callee (procedure) will save registers $s0-$s7 starting at memory address FF000000 when it’s called, and restore the registers before it returns • This may not be necessary for all procedures

Procedure Calls • So here’s our procedure… factorial: lui $t1,0xFF00 sw $s0,0($t1) sw $s1,4($t1) … sw $s7,28($t1) move $s0,$a0 # new instruction move $s1,$a0 loop0: bgtz $s0,loop1 b out loop1: mult $s0,$s1 mflo $s1 subi $s0,$s0,1 b loop0 out: move $v0,$s1 lw $s0,0($t1) # load back registers … jr $31

Procedure Calls • In order to call our procedure… • load a value into $a0 • bal factorial • look in $v0 for the result!

I/O • Doing I/O with SPIM is also described in Appendix A, but we’re going to use the system calls that are set up for us to do I/O…

I/O • SPIM provides some operating system services through the syscall instruction • To use the system calls: • load system call code (from pg. a49) into $v0 and argument into $a0… • return values in $v0 (or $f0 for fp results)

Example I/O str: .asciiz “the answer = “ .text li $v0,4 la $a0, str syscall li $v0,1 la $a0,5 syscall (Refer to page A-49)

Chapter 3: Instructions