For two days I've been putzing around trying to get FDS to work. I've read the docs I could find over and over and I thought I had a decent understanding of how it works, but when I impliment it, it just ain't working. I constantly get various Disk errors when the BIOS is trying to read the disk header. In my latest attempt, I get no error, but the BIOS simply locks up and forever loops at a "now loading" screen.
I've been using BT's doc as my primary reference, with other docs to mainly fill in the gaps (IRQ timer mainly, even though that's low on my priority list right now -- i just want to get the games loaded and running).
Here's my current understanding. If ANYONE can clarify the grey areas, make any kind of correction, or throw any ideas out on the table... it'd be GREATLY appreciated:
- BIOS at $E000-FFFF. Always -- no exceptions.
- 32k RAM (flushed on hard reset) at $6000-$DFFF. Always. No swapping, no banks -- just a flat out big hunk of RAM.
- Each Disk side is 65500 bytes, starting immediately after the 16-byte .fds header. ie: Disk 1- Side A @ 0x00010, Disk 1- Side B @ 0x0FFEC, Disk 2- Side A @ 0x1FFC8, etc
- Two seperate and completely independent IRQ generators on the FDS -- one for use by the game (16-bit decrementing CPU cycle counter set by $4020/1/2) -- and one which is used to be alerted when the drive is ready for another transfer (enabled by $4025.7)
- $4024 and $4031 do not write/read the disk directly -- rather they act as a buffer to hold the next byte to be written/read
now this is where it gets hairy -- since docs have conflicting info... and BT's doc isn't exactly crystal clear:
- Clearing $4025.0 will turn off the drive motor. It can't be turned on again until this bit is set again.
- Clearing $4025.1 will turn ON the drive motor, unless $4025.0 is clear. Setting $4025.1 will not turn off the motor if it's already on (you must clear $4025.0 to turn it off). To quote BT's doc: "When deactivated, disk drive motor stays on until disk head reaches most inner track of disk". Considering he said a value of 0 is "activated", I'd assume "deactivated" means 1.
- However, setting $4025.1 will disrupt disk I/O somehow. No doc gives any explaination as to how/why... and I don't really understand this part at all.
- If the drive motor is off, it resets to the start of the disk -- ie when turned back on, the next byte to be transfered will be at offset $0000 on the disk.
- $4030.1 returns the value of the byte transfer flag.
- $4032.1 is cleared only when the drive position is reset to 0 (motor turned off?). It is set when the drive reaches the end of the disk. ??? Or when $4025.1 is set???
- If the drive motor is on and disk IO is enabled (and of course if a disk is in the drive), the following happens: a) every ?100? CPU cycles and b) if the byte transfer flag is clear:
1) If in write mode, value in $4024 gets written to disk. Else if in read mode, value gets read from disk and gets put in $4031.
2) Byte Transfer flag gets set (IRQ tripped if enabled). This is the only way to set the Xfer flag?
- Only way to clear byte transfer flag (so that another byte can be read/written) is to read $4031, $4030, or write to $4024. (and maybe turn the motor off? although no doc says that)
That's about it! That's the most sense I could make out of BTs doc. The biggest concern of mine (and I'm willing to bet the source of all my troubles) is $4032.1. When exactly is this bit set/clear?
Anyway, like I said, any sort help would be greatly appreciated. Thanks in advance.
I've been using BT's doc as my primary reference, with other docs to mainly fill in the gaps (IRQ timer mainly, even though that's low on my priority list right now -- i just want to get the games loaded and running).
Here's my current understanding. If ANYONE can clarify the grey areas, make any kind of correction, or throw any ideas out on the table... it'd be GREATLY appreciated:
- BIOS at $E000-FFFF. Always -- no exceptions.
- 32k RAM (flushed on hard reset) at $6000-$DFFF. Always. No swapping, no banks -- just a flat out big hunk of RAM.
- Each Disk side is 65500 bytes, starting immediately after the 16-byte .fds header. ie: Disk 1- Side A @ 0x00010, Disk 1- Side B @ 0x0FFEC, Disk 2- Side A @ 0x1FFC8, etc
- Two seperate and completely independent IRQ generators on the FDS -- one for use by the game (16-bit decrementing CPU cycle counter set by $4020/1/2) -- and one which is used to be alerted when the drive is ready for another transfer (enabled by $4025.7)
- $4024 and $4031 do not write/read the disk directly -- rather they act as a buffer to hold the next byte to be written/read
now this is where it gets hairy -- since docs have conflicting info... and BT's doc isn't exactly crystal clear:
- Clearing $4025.0 will turn off the drive motor. It can't be turned on again until this bit is set again.
- Clearing $4025.1 will turn ON the drive motor, unless $4025.0 is clear. Setting $4025.1 will not turn off the motor if it's already on (you must clear $4025.0 to turn it off). To quote BT's doc: "When deactivated, disk drive motor stays on until disk head reaches most inner track of disk". Considering he said a value of 0 is "activated", I'd assume "deactivated" means 1.
- However, setting $4025.1 will disrupt disk I/O somehow. No doc gives any explaination as to how/why... and I don't really understand this part at all.
- If the drive motor is off, it resets to the start of the disk -- ie when turned back on, the next byte to be transfered will be at offset $0000 on the disk.
- $4030.1 returns the value of the byte transfer flag.
- $4032.1 is cleared only when the drive position is reset to 0 (motor turned off?). It is set when the drive reaches the end of the disk. ??? Or when $4025.1 is set???
- If the drive motor is on and disk IO is enabled (and of course if a disk is in the drive), the following happens: a) every ?100? CPU cycles and b) if the byte transfer flag is clear:
1) If in write mode, value in $4024 gets written to disk. Else if in read mode, value gets read from disk and gets put in $4031.
2) Byte Transfer flag gets set (IRQ tripped if enabled). This is the only way to set the Xfer flag?
- Only way to clear byte transfer flag (so that another byte can be read/written) is to read $4031, $4030, or write to $4024. (and maybe turn the motor off? although no doc says that)
That's about it! That's the most sense I could make out of BTs doc. The biggest concern of mine (and I'm willing to bet the source of all my troubles) is $4032.1. When exactly is this bit set/clear?
Anyway, like I said, any sort help would be greatly appreciated. Thanks in advance.
