FDC/DMA hardware interface
The hardware interface of
FDC is composed of two adresses for
FDC/
DMA interface and three adresses for the
DMA pointer, which is split into 3 bytes. Please note that selection of drive and side is done into the
SoundGenerator? register #14.
DMA pointer
The
DMA pointer is set by the programmer to tell the
DMA where to read data to be written on disk (
FDC write commands) and where to write data that is to be read from disk (
FDC read commands). This 24-bit pointer is split into 3 separate bytes as follows:
| Address |
Data format |
Access |
Access to |
| 0x00FF8609 |
Byte |
R/W |
DMA pointer higher byte |
| 0x00FF860B |
Byte |
R/W |
DMA pointer medium byte |
| 0x00FF860D |
Byte |
R/W |
DMA pointer lower byte |
Warning
When setting the
DMA pointer, the lower byte should be set first, the higher byte last.
Caveats
As you can guess, the
DMA pointer is limited to 24 bits, so
DMA can't access RAM that would be above the 16MB limit. This means that in case you machine has TT RAM (also called
alternate memory) above 16MB, then a specific driver will be necessary for your applications to be able to put data from disk to TT RAM. This means that older application accessing the
FloppyDrive directly through this hardware interface (without using the operating system, which is absolutely not recommended) won't be able to load data from disc to TT RAM.
Drive and Side selection
The selection of side 1 or 2 and drive
A: or
B: is done in the rgister #14 of the
SoundGenerator? chip. To access this register, you first have to write
0x0E at
0x00FF8800.
| Bit |
Function |
Meaning |
| #0 |
Side selection |
1: Side 1 selected, 0: Side 2 selected |
| #1 |
Drive #0 selection (internal A: drive) |
1: Drive #0 not selected, 0: Drive #0 selected |
| #2 |
Drive #1 selection (external B: drive) |
1: Drive #1 not selected, 0: Drive #1 selected |
Caveats
Be carefull that these bits are set to
1 by default, which is the usual unselected state for
FloppyDisc? signals. Thus, selection is made when clearing the considered bits to
0.
FDC/
DMA interface is composed of two addresses used to control the
DMA and access the various
FDC registers.
| Address |
Access |
Available bits |
Access to |
| 0x00FF8604 |
Read |
FEDCBA98 76543210
________ XXXXXXXX |
One of the FDC registers (depending on AA bits),
or the Sector Count register (if C bit is set to 1) |
| 0x00FF8604 |
Write |
FEDCBA98 76543210
________ XXXXXXXX |
One of the FDC registers (depending on AA bits),
or the Sector Count register (if C bit is set to 1) |
| 0x00FF8606 |
Read |
FEDCBA98 76543210
________ _____RSE |
Status Register for FDC and DMA |
| 0x00FF8606 |
Write |
FEDCBA98 76543210
_______W FD_CHAA_ |
Control Register for FDC and DMA |
FDC/DMA status bits
Bits that are read from the Status Register at 0x00FF8606 have the following meaning:
| Bit |
Meaning |
| #0: R |
Data Request inactive |
| #1: S |
Sector count status |
| #2: E |
DMA Error status |
FDC/DMA control bits
When writing in the Control Register at 0x00FF8606, bits have the following meaning:
| Bit |
Meaning |
| #0: Reserved |
None |
| #1 & #2: AA |
FDC register selection when accessing 0x00FF8604. See below for the meaning of these two bits. |
| #3: H |
(_CS) H=1: Hard drive / H=0: Floppy disk |
| #4: C |
C=1: Select Sector Count register / C=0: Select FDC/DMA registers |
| #5: Reserved |
None |
| #6: D |
D=1: DMA active / D=0: DMA inactive |
| #7: F |
(_DRQ) F=1: DMA works with Floppy drive / F=0: DMA works with hard drive |
| #8: W |
W=1: DMA Writes to memory / W=0: DMA reads from memory |
When
C=0 (bit #4 of the Control Register cleared), bits
AA of the Control Register let you select which
FDC register will be accessed at
0x00FF8604. When
AA=00, two different registers are accessed when reading or writing. Available registers are listed below:
| Value |
Access |
Accessed register |
| AA=00 |
Read |
Status Register |
| AA=00 |
Write |
Command Register |
| AA=01 |
Read/Write |
Track Register |
| AA=10 |
Read/Write |
Sector Register |
| AA=11 |
Read/Write |
Data Register |