IOUtools/NVRAM.md

NVRAM Structure

This documents the IOU NVRAM structure. It is based on the information in dynamips’ fs_nvram.h, see https://github.com/GNS3/dynamips/blob/master/common/fs_nvram.h

Only the first half of the NVRAM is used, the second half is zero filled.

The structure is almost identical to the Dynamips NVRAM layout. The major difference is the missing NVRAM header in IOU.

Layout of first Half
Startup Header
Startup Configuration
Padding
Private Header
Private Configuration
Free Space
File Area

All data is stored in big endian order, high byte first.

Startup Header

Off	Len	Usage
0	2	Magic 0xABCD
2	2	Format, 1 = raw, 2 = compressed
4	2	Checksum
6	2	IOS version, e.g. 0x0F04 = 15.4
8	4	Start address
12	4	End address
16	4	Length
20	4	??? 0
24	4	??? 0 = raw, 1 = compressed
28	2	??? 0 = raw, 1 = compressed
30	2	??? 0
32	4	uncompressed len, raw = 0

Total 36 bytes

The checksum is basically the sum of all 2-byte-words in the first half of the NVRAM. For further details have a look into iou_import, function checksum.

Startup Configuration

In raw format the configuration is stored unchanged. In compressed format, the configuration is packed the same way as in the unix compress program, see http://en.wikipedia.org/wiki/Compress . It uses the LZW algorithm.

Padding

The startup-config is padded to an aligment of 4. In IOS <= 15.0 or when the NVRAM size is at least 128 KByte, if padding takes place, an additional 4 byte padding is added.

Private Header

Off	Len	Usage
0	2	Magic 0xFEDC
2	2	Format, only 1 = raw
4	4	Start address
8	4	End address
12	4	Length

Total 16 bytes

Private Configuration

The private configuration is always stored in raw format.

File Area

The files are growing from the end of the area down to lower offsets. Each file block is 1024 bytes long and starts with the magic number 0xDCBA. An unused/erased block is all zero.

File Sector

Off	Len	Usage
0	2	Magic 0xDCBA
2	2	Next sector
4	2	Flags, bit 0=start, 1=end, 2=no RW
6	2	Length of data in this block
8	24	File name, NUL terminated
32	992	Data

Total 1024 bytes