a) Read sectors
b) Write sectors
c) Format track
d) Restore to track 0
e) Seek
5) To perform these functions the following operations are needed
a) Select a drive
b) Start spindle motor
c) Time motor startup
d) Calculate track/head/sector
e) Seek/restore to track
f) Verify track no. - optional
g) Time head settling
h) Issue '765 command
i) Get '765 status
j) Setup pseudo DMA under FIQ
k) Read sector(s) in a track
l) Write sector(s) in a track
m) Format a track
n) Implement command timeout
o) Set bit rate (125/250/500/1000 Kbps)
p) Reset '765
q) Read/reset disk changed
r) Count index pulses
Note that because Acorn L,D and E format disks start from sector no. 0 it will not be possible to use the "multitrack" function of the '765 controller. In addition because L format disks use 16x256 byte sectors per track (ECMA format - no IAM field), the '765 will not be able to format these disks exactly because of the IBM compatible IAM being written. Still it may be possible, like BEEBDOS, to format these disks by overwriting the IAM with the last sector. Some thought needs to go into gap sizes etc.
6) In order to sequence these operations a finite state system (state machine) will be used. It is essential to arbitrate access to the state machine (by foreground/background accesses) because it is a single threaded construct. The objective of the machine is to sequence motor start, head settling and command completion.
7) The following events will be recognized:
a) TimeOut
b) Index pulse
c) FDC interrupt (Command done, seek done, ready)
d) Disk changed, interrupt or polled?
7a) The following requests will be recognized:
a) Seek request(track 0..79 or any)
b) Reset request, reset '765 restore head
c) Lock drive
d) Unlock drive
7b) The following messages will be passed to pending commands:
a) Track(0..79 or unknown)
b) TimeOut
c) DiskChanged
d) FDC interrupt
8) The following states will exist
a) Drives idle
b) Motor started, motor speeding up
c) Motor ready, motor up to speed
d) Seeking, head being stepped
e) Settling, head settling after seek
f) Drive ready
g) Drive busy, read/write pending. Is this necessary?
9) What about retries? Handled by command processor. 9a) Retry algorithm?
a) Retry operation (disable VIDC if over/underrun)
b) Step out, step in, retry operation
c) Restore, seek, retry operation
10) Scatter list updated after each transfer request (1 track max)
11) A general disk command processor entry point, passed a pointer to a disk control block (DCB) structured as follows:
+00 BufferAddress & 1 ; Data buffer pointer
+04 BufferSize & 1 ; Data buffer size +08 StaticBase & 1 ; R12 for post routine+0c PostRoutine & 1 ; Address to call on command completion +10 CommandStatus & 1 ; -1 = pending, 0= no error else an error ptr
+14 TimeOut & 1 ; Command timeout in centiseconds, 0= none +18 Retries = 1 ; Retries, 0= none +19 ParamCount = 1 ; 0..13 bytes. Bit7 = NoWait command+1a Command = 1
+28 Results = 08 ; 1..8 bytes, command specific +30 Link & 1 ; Reserved, ptr to next DCB+34 MessageHandler & 1 ; Reserved, ptr to message handler for this command
+38 Reserved = 08 ; Reserved for driver
Total size 64 bytes
In addition to device specific commands a "Cancel" function is necessary to cancel a pending DCB. In this case "BufferAddress" -> DCB to cancel.
Command processor action:
a) Check command valid, add MessageHandler address
b) Queue the DCB (sorted by track), status= pending
c) Call the DCB message handler with MessageIdle
d) MessageIdle will result in handler generating SeekRequest(n) to state m/c
e) State system starts drive motor and performs seek if required
f) State system generates EventTrack(n)
g) Message handler claims event if track ok and issues command
h) State system generates EventResult
i) Message handler reads '765 status
j) Command completed, post routine called
k) MessageIdle sent to pending DCB's, next DCB starts processing
Under interrupt the following happens:
a) Arranged like '765 command block.
b) Implied seek is bit 7 set of 1st parameter
Supported commands:
a) Read/write data/deleted data inc. multitrack
b) Read track
c) Read ID
d) Format track
e) Scan
f) Seek, recalibrate -> state system
g) Sense interrupt status
h) Sense drive status
i) Specify
j) Invalid
Determine source of FIQ (DRQ/IRQ), branch to Flp765IRQ if IRQ
a) Read block - FlpFIQread
b) Write block - FlpFIQwrite
c) Verify (read and throw away) - FlpFIQverify
d) All start:
LDRB Temp, [IOCfiq, #IOCFIQSTA] ; Read FIQ status from IOC
TSTS Temp, IoDrqBit ; DRQ?
BEQ Flp765IRQ ; No then jump, check IRQ
LDRB Temp, [FDCdack], # 1 ; Read data with DACK/TC, FDCdack++
Call DCB's with MessageIRQ, this allows DCB's to get results
Queue next DCB from scatter list if required
Downgrade FIQ to IRQ if no DCB's
Release FIQ to FileCore if all commands done.
Read '765 status register. If RQM and DIO are set then read results
else issue sense interrupt to get ST0