#

#### Technical details

The Disc Parameter Blocks (DPBS) of both drives, the Allocation Vector of drive A (ALVA - a representation of disc usage) and the Check Sum of drive A (CKSA - a representation of directory usage) remain at their normal addresses but the ALVB is located at &BEOO and the CKSB at &BECO. These CKSs and ALVs allows for a maximum of 184K and 64 directory entries in drive A, and 800K and 256 directory entries in drive B. If larger values are required then the CKS or ALV concerned will have to be moved.

The different formats are recognised by their sector numbers. The standard formats for drive A use the same sector numbers as normal. The first sector numbers of the different formats are:

Drive A |

- PCW = &01
- Buffer = &11
- System = &41
- Data = &C1
- Other = &81

Drive B

- D1 = &01
- D10 = &11
- D2 = &21
- D20 = &31

- D40 = &51
- D80 = &71
- O = &81

Double sided discs are assumed to be single step and flip-sided, but the sidedness and stepping can be changed by function 1, requesting the "Other" format types.

Sidedness of three kinds are recognised:

- Flip-sides: Logical tracks 0-159 on alternate sides of the disc, from physical track 0-79.
- Up and Over A: Logical tracks 0-79 on side 0 of the disc. and logical tracks 80-159 on side 1.
- Up and Over B: this is the reverse of normal Up and Over on the second side, so that both sides are identical.

The table below may (or may not) make this clearer. If in doubt, use Flip sides, which is the fastest and commonest format. If you are trying to read a strange disc and Flip sides produces nonsense, then try Up and Over A, which is much more common than B.

Sidedness | Logical track no | Physical track no | Side
--- | --- | --- | ---
Flip sides | 0 | 0 | 0
1 | 0 | 1
2 | 1 | 0
3 | 1 | 1
159 | 80 | 1
Up and Over A | 0 | 0 | 0
1 | 1 | 0
79 | 79 | 0
80 | 79 | 1
159 | 0 | 0
Up and Over B | 0 | 0 | 0
79 | 79 | 0
80 | 0 | 1
159 | 79 | 0