1. CSE 410, Spring 2008 Computer Systems
Testing and Branching
CSE 410, Spring 2008
Computer Systems
5/20/2019
cse decisions © Perkins, DWJohnson & University of Washington

2. Reading and References
Computer Organization and Design
Section 2.5, Logical Operations
Section 2.6, Instructions for Making Decisions
(CD) Section A.9, SPIM
(CD) Section A.10 through page A-50, MIPS R2000
We’ll see a version of this chip as a guest.
5/20/2019
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3. Goto Considered Harmful
“Oh what a tangled web we weave, When first we practice to deceive!”
Sir Walter Scott
“We’re not lost. We’re locationally challenged”
Ford
“That code looks like spaghetti!”
anon
Branching in assembly language can turn your program into a rat’s nest that cannot be debugged
Keep control flow simple and logical
SESE blocks
Use comments describing the overall logic
(if, while, for, … pseudo-code is great!)
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4. Conditional Branch
...
A change in the program’s flow of control that depends on some condition
...
... ?
yes no
...
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5. Unconditional Branch (or, jump)
...
j, jal, jr
...
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6. Determinism (1 path) V.S. Nondeterminism (n-paths)
T/F: a program’s execution can be exactly determined ahead of time if no branches are used
T/F: a program’s execution can be exactly determined ahead of time if only unconditional branches are used
T/F: a program’s execution can be exactly determined ahead of time if conditional branches are used
This is what differentiates us from a calc watch!
What if we had an if that said:
if ( timeOfDay % 2 == 0 ) do path 1; else do path 2;
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7. Blocks A chunk of code, an atomic block
SESE – single entry, single exit
At most one label (entry point) in the beginning
At most one label (exit point) at the end
Analagous to a function call with only one return as the last instruction.
5/20/2019
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8. Branch instructions Branch instructions are I-format instructions
op code field
two register fields
16-bit offset field
Simplest branches check for equality
beq $t0, $t1, address
bne $t0, $t1, address
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9. if (i==j) then a=b; Assume all values are in registers
Note that the test is inverted compared to if!
Why?
# $t0=i, $t1=j, $s0=a, $s1=b
bne $t0, $t1, skip
move $s0, $s1
skip:
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cse decisions © Perkins, DWJohnson & University of Washington

10. while (s[i]==k) i = i+j;
# $s0=addr(s), $v1=i, $a0=k, $a1=j
loop:
sll $v0,$v1,2 # v0 = 4*i
addu $v0,$s0,$v0 # v0 = addr(s[i])
lw $v0,0($v0) # v0 = s[i]
addu $v1,$v1,$a1 # i = i+j
beq $v0,$a0,loop # loop if equal
subu $v1,$v1,$a1 # i = i-j
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11. for (i=0; i<10; i++) s[i] = i;
# $s0=addr(s), $t1=i
move $t1,$zero # i = 0
loop:
sll $t0,$t1,2 # t0 = i*4
addu $t0,$s0,$t0 # t0 = addr(s[i])
sw $t1,0($t0) # s[i] = i
addu $t1,$t1,1 # i++
slt $t0,$t1,10 # if (i<10) $t0=1
bnez $t0,loop # loop if (i<10)
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12. How do we encode the destination?
op code
base reg
src/dest
offset or immediate value
6 bits
5 bits
5 bits
16 bits
Calculating the destination address
4*(the 16-bit offset value)
is added to the Program Counter (PC, our current loc)
The offset is a word offset in this case
Why?
The base register is always the PC, so we don’t need to specify it in the instruction
Covers a range of 216 words (64 KW)
Is this enough?
Governing branching and looping is the principle of locality (both spatial and temporal)
5/20/2019
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13. Comparison instructions
For comparisons other than equality
slt : set less than
sltu : set less than unsigned
slti : set less than constant value
sltiu : set less than unsigned constant
set t0 to 1 if t1<t2
slt $t0, $t1, $t2
Usually paired with a branch (below not optimized)
#if( a < b ) then go somewhere, else go somewhere different
Slt $t0, $t1, $t2 #a = $t1, b = $t2
Beq $t0, $zero, Else-Label #if == 0, ! ( a < b)
#logic for if a < b
J End-Label
Else-Label:
#logic for if ! (a < b)
End-Label:
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14. Pseudo-instructions The assembler is your friend Original code:
It will build instruction sequences for you
It deals with address/label conversions for you
Original code:
bge $a0,$t1,end # if a0>=t1 skip
Actual instructions:
slt $at,$a0,$t1 # if a0<t1 at=true
beq $at,$0,end # skip if at==false
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15. Jump Instructions
Jump instructions provide longer range than branch instructions
26-bit word offset in J-Type instructions
j : jump
jal : jump and link (store return address)
32-bit address in register jumps
jr : jump through register
jalr : jump through register and link
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16. J-Type fields
op code
word offset
6 bits
26 bits
The word offset value is multiplied by 4 to create a byte offset (quick sll by ?)
the result is 28 bits wide
Then concatenated with top 4 bits of PC to make a 32 bit destination address
Why?
Offset implies this, but is the 26 bits absolute or relative?
What if we need to form an absolute 32-bit address to span the whole memory space (in this example, 4GB)
Would this J-Type instruction suffice?
5/20/2019
cse decisions © Perkins, DWJohnson & University of Washington

17. Important Jumps Jump and link (jal) Jump through register (jr)
call procedure and store return address in $ra
Jump through register (jr)
return to caller using the address in $ra
We will talk about procedure calls in excruciating detail shortly
Crucio! (Latin: crux, “I torture”)
5/20/2019
cse decisions © Perkins, DWJohnson & University of Washington