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Marketable Securities
 3 Months Ended
Dec. 30, 2017
Marketable Securities [Abstract]  
Marketable Securities
Marketable Securities
The amortized cost and fair value of marketable securities as of December 30, 2017 and September 30, 2017 were as follows:

December 30, 2017

Amortized cost
 
Gross unrealized gains
 
Gross unrealized losses
 
Fair value
 
(in thousands)
Certificates of deposit
$
221

 
$
—

 
$
(1
)
 
$
220

Corporate notes/bonds
48,268

 
—

 
(311
)
 
47,957

US government agency securities
2,401

 
—

 
(11
)
 
2,390


$
50,890

 
$
—

 
$
(323
)
 
$
50,567


September 30, 2017

Amortized cost
 
Gross unrealized gains
 
Gross unrealized losses
 
Fair value
 
(in thousands)
Certificates of deposit
$
240

 
$
—

 
$
—

 
$
240

Corporate notes/bonds
47,811

 
2

 
(140
)
 
47,673

US government agency securities
2,407

 
—

 
(5
)
 
2,402


$
50,458

 
$
2

 
$
(145
)
 
$
50,315


Our investment portfolio consists of certificates of deposit, commercial paper, corporate notes/bonds and government securities that have a maximum maturity of three years. The longer the duration of these securities, the more susceptible they are to changes in market interest rates and bond yields. All unrealized losses are due to changes in market interest rates, bond yields and/or credit ratings.
We review our investments to identify and evaluate investments that have an indication of possible impairment. We concluded that, at December 30, 2017, the unrealized losses were temporary. The following tables summarize the fair value and gross unrealized losses aggregated by category and the length of time that individual securities have been in a continuous unrealized loss position as of December 30, 2017 and September 30, 2017.
 
December 30, 2017
 
Less than twelve months
 
Greater than twelve months
 
Total
 
Fair Value
 
Gross unrealized loss
 
Fair Value
 
Gross unrealized loss
 
Fair Value
 
Gross unrealized loss
 
(in thousands)
Certificates of deposit
$
220

 
$
(1
)
 
$
—

 
$
—

 
$
220

 
$
(1
)
Corporate notes/bonds
22,673

 
(183
)
 
25,284

 
(128
)
 
47,957

 
(311
)
US government agency securities
—

 
—

 
2,390

 
(11
)
 
2,390

 
(11
)
 
$
22,893

 
$
(184
)
 
$
27,674

 
$
(139
)
 
$
50,567

 
$
(323
)

 
September 30, 2017
 
Less than twelve months
 
Greater than twelve months
 
Total
 
Fair Value
 
Gross unrealized loss
 
Fair Value
 
Gross unrealized loss
 
Fair Value
 
Gross unrealized loss
 
(in thousands)
Certificates of deposit
$
240

 
$
—

 
$
—

 
$
—

 
$
240

 
$
—

Corporate notes/bonds
15,254

 
(43
)
 
28,885

 
(97
)
 
44,139

 
(140
)
US government agency securities
—

 
—

 
2,402

 
(5
)
 
2,402

 
(5
)
 
$
15,494

 
$
(43
)
 
$
31,287

 
$
(102
)
 
$
46,781

 
$
(145
)


The following table presents our available-for-sale marketable securities by contractual maturity date as of December 30, 2017 and September 30, 2017.

December 30, 2017
 
September 30, 2017

Amortized cost
 
Fair value
 
Amortized cost
 
Fair value
 
(in thousands)
 
(in thousands)
Due in one year or less
$
20,025

 
$
19,962

 
$
18,274

 
$
18,244

Due after one year through three years
30,865

 
30,605

 
32,184

 
32,071


$
50,890

 
$
50,567

 
$
50,458

 
$
50,315