A FAT (file allocation table) based file system is being used and the total overhead of each entry in the FAT is 4 bytes in size. Given a $100 \times 10^6$ bytes disk on which the file system is stored and data block size is $10^3$ bytes, the maximum size of a file that can be stored on this disk in units of $10^6$ bytes is _______.

1. Total number of disk blocks:

   $$\text{Number of blocks} = \frac{\text{Total disk size}}{\text{Block size}} = \frac{100 \times 10^6\text{ bytes}}{10^3\text{ bytes}} = 10^5\text{ blocks}$$

2. Total size of FAT: Since there is one entry per disk block in the File Allocation Table:

   $$\text{FAT size} = 10^5 \times 4\text{ bytes} = 4 \times 10^5\text{ bytes} = 0.4 \times 10^6\text{ bytes}$$

3. Maximum space available for data (file storage):

   $$\text{Available data space} = \text{Disk size} - \text{FAT size}$$

   $$\text{Available data space} = 100 \times 10^6 - 0.4 \times 10^6 = 99.6 \times 10^6\text{ bytes}$$

In units of $10^6$ bytes, the maximum size is 99.6.