hank you for purchasing our military$-$grade, nuclear bomb$-$proof hard disks, built with patented NASA

technologies. Each of the disks in front of you consists of $256$ blocks, and each block has $256$ bytes, coming

to a total of $256\backslash$times $256=65,536$ bytes per disk. Since you bought $16$ disks, the combined capacity is $16\backslash$times

$65,536=1,048,576$ bytes $=1$ MB$.$ We provide you with a device driver with a single function that you can use

to control the disks.

int jbod$\_$operation$($uint$32\_$t op$,$ uint$8\_$t $*$block$)$;

This function returns $0$ on success and $-1$ on failure. It accepts an operation through the op parameter,

the format of which is described in Table $1,$ and a pointer to a buffer. The command field can be one of the

following commands, which are declared as a C enum type in the header that we have provide to you:

$1.$ JBOD$\_$MOUNT: mount all disks in the JBOD and make them ready to serve commands. This is the first

command that should be called on the JBOD before issuing any other commands; all commands before

it will fail. When the command field of op is set to this command, all other fields in op are ignored by

the JBOD driver. Similarly, the block argument passed to jbod$\_$operation can be NULL.

$1$

$2.$ JBOD$\_$UNMOUNT: unmount all disks in the JBOD. This is the last command that should be called on

the JBOD; all commands after it will fail. When the command field of op is set to this command, all

other fields in op are ignored by the JBOD driver. Similarly, the block argument passed to jbod$\_-$

operation can be NULL.

$3.$ JBOD$\_$SEEK$\_$TO$\_$DISK: seeks to a specific disk. JBOD internally maintains an I$/$O position, a tuple

consisting of $\{$CurrentDiskID$,$ CurrentBlockID$\},$ which determines where the next I$/$O operation will

happen. This command seeks to the beginning of disk specified by DiskID field in op$.$ In other words, it

modifies I$/$O position: it sets CurrentDiskID to DiskID specified in op and it sets CurrentBlockID to $0.$

When the command field of op is set to this command, the BlockID field in op is ignored by the JBOD

driver. Similarly, the block argument passed to jbod$\_$operation can be NULL.

$4.$ JBOD$\_$SEEK$\_$TO$\_$BLOCK: seeks to a specific block in current disk. This command sets the CurrentBlockID in I$/$O position to the block specified in BlockID field in op$.$ When the command field of op

is set to this command, the DiskID field in op is ignored by the JBOD driver. Similarly, the block

argument passed to jbod$\_$operation can be NULL.

$5.$ JBOD$\_$READ$\_$BLOCK: reads the block in current I$/$O position into the buffer specified by the block

argument to jbod$\_$operation. The buffer pointed by block must be of block size, that is $256$ bytes.

After this operation completes, the CurrentBlockID in I$/$O position is incremented by $1$; that is$,$ the next

I$/$O operation will happen on the next block of the current disk unless you specify a new DiskID or

BlockID. When the command field of op is set to this command, all other fields in op are ignored by the

JBOD driver.

$6.$ JBOD$\_$WRITE$\_$BLOCK: writes the data in the block buffer into the block in the current I$/$O position.

The buffer pointed by block must be of block size, that is $256$ bytes. After this operation completes,

the CurrentBlockID in I$/$O position is incremented by $1$; that is$,$ the next I$/$O operation will happen on the

next block of the current disk unless you specify a new DiskID or BlockID. When the command field of

op is set to this command, all other fields in op are ignored by the JBOD driver.

After you finished your onboarding session with HR and enjoyed the free lunch with your new colleagues,

you received the following email from the manager of the team.