The 24xx256 is an I2C EEPROM and an easy way to extend the memory of your mircoc-controller by 256kbit, which is 32kb (32768 bytes). The 24LC256 version can be bought from SparkFun for $2 and it is very easy to program.

It is good to read the complete 24xx256 Datasheet , but here is a summary:

Byte 7 6 5 4 3 2 1 0 Function
1st
1
0
1
0
A2
A1
A0
R/W
2nd
ignored
Addr<14:8>
3rd
Addr<7:0>

There is a great tutorial on the dangerousprototypes site.

Setup

HiZ> m 4 3 1
I2C (mod spd)=( 0 2 )
Ready
I2C> W
Power supplies ON
I2C> P
Pull-up resistors ON
I2C>

Writing Data

I2C> [ 0xa0 0 0 1 3 2 ]
I2C START BIT
WRITE: 0xA0 ACK
WRITE: 0x00 ACK
WRITE: 0x00 ACK
WRITE: 0x01 ACK
WRITE: 0x03 ACK
WRITE: 0x02 ACK
I2C STOP BIT
I2C>

Reading Data

I2C> [ 0xa0 0 0 ] [ 0xa1 r:3 ]
I2C START BIT
WRITE: 0xA0 ACK
WRITE: 0x00 ACK
WRITE: 0x00 ACK
I2C STOP BIT
I2C START BIT
WRITE: 0xA1 ACK
READ: 0x01  ACK 0x03  ACK 0x02
NACK
I2C STOP BIT
I2C>