Decisions in C / Assembly Language
Review (1/2)
°In MIPS Assembly Language:
• Registers replace C variables
• One Instruction (simple operation) per line • Simpler is Better
• Smaller is Faster
° Memory is byte-addressable, but lw and sw access one word at a time.
° A pointer (used by lw and sw) is just a memory address, so we can add to it or subtract from it (using offset).
Review (2/2)
°New Instructions:
add, addi, sub, lw, sw
°New Registers:
C Variables: $s0 – $s7
Temporary Variables: $t0 – $t9 Zero: $zero
Overview
° C/Assembly Decisions: if, if-else ° C/Assembly Loops: while, do while,
for
° Inequalities
°C Switch Statement
So Far…
°All instructions have allowed us to manipulate data.
°So we’ve built a calculator.
°To build a computer, we need ability to
make decisions…
°Heads up: pull out some papers and pens, you’ll do some in-class exercises today!
C Decisions: if Statements
° 2 kinds of if statements in C
•if (condition) clause
•if (condition) clause1 else clause2
° Rearrange 2nd if into following:
if (condition) goto L1; clause2;
go to L2; L1: clause1;
L2:
• Not as elegant as if-else, but same meaning
MIPS Decision Instructions
°Decision instruction in MIPS:
•beq register1, register2, L1
•beq is “Branch if (registers are) equal” Same meaning as (using C):
if (register1==register2) goto L1
°Complementary MIPS decision instruction •bne register1, register2, L1
•bne is “Branch if (registers are) not equal” Same meaning as (using C):
if (register1!=register2) goto L1
°Called conditional branches
MIPS Goto Instruction
°In addition to conditional branches, MIPS has an unconditional branch:
j label
°Called a Jump Instruction: jump (or branch) directly to the given label without needing to satisfy any condition
°Same meaning as (using C): goto label
°Technically, it’s the same as: beq $0,$0,label
since it always satisfies the condition.
Compiling C if into MIPS (1/2)
°Compile by hand
if (i == j) f=g+h;
else f=g-h;
°Use this mapping:
f: $s0, g: $s1, h: $s2, i: $s3, j: $s4
(true) i == j
i == j?
(false) i != j
f=g+h
f=g-h
Exit
Compiling C if into MIPS (2/2) (true)
i == j
°Final compiled MIPS code (fill in the blank):
i == j?
(false) i != j
f=g+h
f=g-h
Exit
Compiling C if into MIPS (2/2) (true)
i == j
°Final compiled MIPS code:
i == j?
(false) i != j
f=g+h
f=g-h
beq $s3,$s4,True # branch i==j sub $s0,$s1,$s2 # f=g-h(false) j Fin #gotoFin
True: add $s0,$s1,$s2 # f=g+h (true) Fin:
Note: Compiler automatically creates labels to handle decisions (branches) appropriately. generally not found in HLL code.
Exit
Loops in C/Assembly (1/3)
°Simple loop in C
do {
g = g + A[i];
i = i + j;
} while (i != h);
°Rewrite this as:
Loop:g = g + A[i]; i = i + j;
if (i != h) goto Loop;
°Use this mapping:
g: $s1, h: $s2, i: $s3, j: $s4, base of A:$s5
Loops in C/Assembly (2/3)
°Final compiled MIPS code (fill in the blank):
Loops in C/Assembly (2/3)
°Final compiled MIPS code:
Loop: sll $t1,$s3,2 #$t1= 4*i add $t1,$t1,$s5 #$t1=addr A lw $t1,0($t1) #$t1=A[i] add $s1,$s1,$t1 #g=g+A[i] add $s3,$s3,$s4 #i=i+j
bne $s3,$s2,Loop# goto Loop # if i!=h
Loops in C/Assembly (3/3)
°There are three types of loops in C: •while
•do… while •for
°Each can be rewritten as either of the other two, so the method used in the previous example can be applied to while and for loops as well.
°Key Concept: Though there are multiple ways of writing a loop in MIPS, conditional branch is key to decision making
Inequalities in MIPS (1/5)
°Until now, we’ve only tested equalities (==and!=inC). Generalprogramsneed to test < and > as well.
°Create a MIPS Inequality Instruction: • “Set on Less Than”
•Syntax:slt reg1,reg2,reg3
• Meaning:
if (reg2 < reg3)
reg1 = 1;
else reg1 = 0;
• In computereeze, “set” means “set to 1”, “reset” means “set to 0”.
Inequalities in MIPS (2/5)
°How do we use this? °Compile by hand:
if (g < h) goto Less;
°Use this mapping: g: $s0, h: $s1
Inequalities in MIPS (3/5)
°Final compiled MIPS code (fill in the blank):
Inequalities in MIPS (3/5)
°Final compiled MIPS code:
slt $t0,$s0,$s1 # $t0 = 1 if g
°We could add 3 more instructions, but: • MIPS goal: Simpler is Better
°Can we implement <= in one or more instructions using just slt and the branches?
°What about >?
°What about >=?
°4 combinations of slt & beq/bneq
Inequalities in MIPS (5/5)
°4 combinations of slt & beq/bneq:
slt $t0,$s0,$s1 # $t0 = 1 if g
bne $t0,$0,Grtr # if(g>h) goto Grtr slt $t0,$s0,$s1 # $t0 = 1 if g
slt $t0,$s1,$s0 # $t0 = 1 if g>h
beq $t0,$0,Lteq # if(g<=h) goto Lteq
Immediates in Inequalities
°There is also an immediate version of slt to test against constants: slti
• Helpful in for loops
if (g >= 1) goto Loop
M I P S
C
Immediates in Inequalities
°There is also an immediate version of slt to test against constants: slti
• Helpful in for loops
if (g >= 1) goto Loop
C
Loop: . . . M
slti $t0,$s0,1
# $t0 = 1 if
# $s0<1 (g<1)
I P S
beq $t0,$0,Loop # goto Loop # if $t0==0
# (if (g>=1))
What about unsigned numbers?
°there are unsigned inequality instructions:
sltu, sltiu
°which set result to 1 or 0 depending on
unsigned comparisons
° $s0 = FFFF FFFAhex, $ s1 = 0000 FFFAhex
°What is value of $t0, $t1? °slt $t0, $s0, $s1
°sltu $t1, $s0, $s1
Example: The C Switch Statement (1/3)
°Choose among four alternatives depending on whether k has the value 0, 1, 2 or 3. Compile this C code:
switch (k) {
case 0: f=i+j; break; /* k=0*/ case 1: f=g+h; break; /* k=1*/ case 2: f=g–h; break; /* k=2*/ case 3: f=i–j; break; /* k=3*/ }
Example: The C Switch Statement (2/3)
°This is complicated, so simplify.
°Rewrite it as a chain of if-else statements, which we already know how to compile:
if(k==0) f=i+j;
else if(k==1) f=g+h;
else if(k==2) f=g–h;
else if(k==3) f=i–j;
°Use this mapping:
f: $s0, g: $s1, h: $s2, i: $s3, j: $s4, k: $s5
Example: The C Switch Statement (3/3)
°Final compiled MIPS code (fill in the blank):
Example: The C Switch Statement (3/3)
°Final compiled MIPS code:
bne $s5,$0,L1 # branch k!=0 add $s0,$s3,$s4 #k==0 so f=i+j j Exit # end of case so Exit
L1: addi $t0,$s5,-1 # $t0=k-1
bne $t0,$0,L2 # branch k!=1
add $s0,$s1,$s2 #k==1 so f=g+h j Exit # end of case so Exit
L2: addi $t0,$s5,-2 # $t0=k-2
bne $t0,$0,L3 # branch k!=2 sub $s0,$s1,$s2 #k==2 so f=g-h
j Exit # end of case so Exit
L3: addi $t0,$s5,-3 # $t0=k-3
bne $t0,$0,Exit # branch k!=3
sub $s0,$s3,$s4 #k==3 so f=i-j Exit:
Things to Remember (1/2)
° A Decision allows us to decide which pieces of code to execute at run-time rather than at compile-time.
°C Decisions are made using conditional statements within an if, while, do while or for.
°MIPS Decision making instructions are the conditional branches: beq and bne.
°To help the conditional branches make decisions concerning inequalities, we introduce a single instruction: “Set on Less Than”called slt, slti, sltu, sltiu
Things to Remember (2/2)
°New Instructions: beq, bne
j
slt, slti, sltu, sltiu