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The FDC or Floppy Disc Controller handles the mechanics and transfer of bytes to and from (up to 4) disc drives and the Oric.

The Disk is a relatively large storage medium, many times the memory capacity of the Oric, therefore a scheme is used to allow easy mapping of the disc surface and selecting the drive. Floppy disks will be either single or double sided, and disc drives will also be either single or dual-headed. When using a double-sided in a single-headed drive (most 3“ drives are single-headed), the floppy has to be flipped over in order to read its second side. The FDC can control up to 4 Disc drives, where each one can be a different type. For example, attaching two 3.5”, a 3“ and a 5.25” is quite feasable.

The Disc surface is mapped into Tracks and Sectors.

A Track is simply a concentric ring on the surface of the disc, and each track is split into a number of sectors. The number of tracks, Sectors and size of sectors is left to the programmer.

However 3“ drives are limited to up to 42 tracks per side whilst 3.5” discs may go as high as 80.

Oric disk operating systems usually store 16 or 17 256-byte sectors in each track (Stratsed allows 18 sectors, but it is not as reliable). Randos and BDDOS use 512-byte sectors.

To calculate the number of bytes that can fit on a single disc, we will use this example for a double sided 3.5“ disc.

Their are 2 sides Their are 80 tracks on the surface of one side of the disc. Their are 18 sectors in each track. Each sector is 256 bytes long.

This means that the total capacity of a double sided 3.5” Disc will be 2*80*18*256 or 737,280 byte capacity.

The FDC in the Microdisc system is memory mapped in the conventional I/O area of page 3 in the range $0310 to $0313. Additional I/O registers and latches in the Microdisc electronics are mapped in the $0314-$031B address range.

Each FDC register has a different function when written to or read from.

When written to (ie. STA $0310)

$0310 takes FDC commands

$0311 takes FDC Track Number

$0312 takes FDC Sector Number

$0313 takes FDC Data to place on the disc

$0314 to $0317 takes FDC Control(*2)

The remaining registers have no meaning when written to. *2 All locations in range act as the same register.

When Read from (ie. LDA $0310)

$0310 holds the FDC Status

$0313 holds the FDC Data read from a disc

$0314 to $0317 holds the FDC INTRQ state

$0318 to $031B holds the FDC DRQ state

The remaining registers have no meaning when read from.

For code examples, the following #defines have been used

        #define fdc_Command   $0310
        #define fdc_Status    $0310
        #define fdc_Track     $0311
        #define fdc_Sector    $0312
        #define fdc_Data      $0313
        #define fdc_Control   $0314
        #define fdc_DRQState  $0318

And now each register in closer detail

Generally the top 3 bits (B5 to B7) of this register define the Command whilst the lower 5 bits (B0 to B4) provide settings for the command used.

Flag Summary

This 8-bit register holds the track number of the current Read/Write head position. It is incremented by one every time the head is stepped in (towards track 79) and decremeted by one when the head is stepped out (towards track 00). The contents of the register are compared with the recorded track number in the ID field during disk Read, Write and Verify operations. This Register should not be loaded when the device is busy.

This 8-bit register holds the address of the desired sector position. The contents of the register are compared with the recorded sector number in the ID field during disk Read or Write operations. This register should not be loaded when the device is busy.

This 8-bit register is used as a holding register during Disk Read and Write operations. In Disk Read operations the assembled data byte is transferred in parallel to the Data Register from the Data Shift Register. In Disk Write operations information is transferred in parallel from the Data Register to the Data Shift Register.

When executing the Seek command the Data Register holds the address of the desired Track position.

The FDC contains its own interrupt controller; this bit enables or disables connection of the FDC's interrupt line to the CPU's. Command completion can be detected in a number of ways.

This switches in (1) or out(0) the internal BASIC ROM. Switching out the internal ROM will expose the top 16K RAM of ORIC memory. This usually holds the Disc operating system (Sedoric).

This selects the data seperator Clock divisor. This bit should always be set to 1.

Selects Double(0) or Single(1) Density Disc. It is generally assumed that most disks are double density so this bit should be set to 0.

Selects the first Side (0) or the other Side (1). For 3.5”, the disc cannot be physically flipped. However with some older drives like the 5.25“, their is no way the FDC can detect which is the default side.

Forms a 2 bit number ranging 0 to 3 which selects the Drive number. Drive 0 is the master drive.

Selects the Internal RANDOS Disc Operating System(0) or switches it out(1). When enabled it will overlay $E000 to $FFFF of ORIC RAM, however the internal BASIC ROM will always take presedent.

The FDC Status Register returns the status of the last operation performed. Each Bit represents a flag of a disc aspect but not all bits are affected by all commands as shown below.

* the Microdisc connects the FDC's RDY line directly to +5v via a pull-up resistor. Therefore drives appear to be ready at all times.

This reflects the FDC's INTRQ line which will be reset when the command has finished or when status is read. It is not affected by the value written to bit 0 of $0314.

When a byte is ready to be read or written Bit 7 of $0318 is Reset.