Reading Packets
and ReadRest routines to read the data. Your protocol handler calls
protocol handler and the ReadPacket and ReadRest routines.
Note: Because an Ethernet protocol handler must read from and write to
the CPU's registers, you cannot write a protocol handler in high- level
language such as Pascal or C.
How the .ENET Driver Calls Your Protocol Handler
You can provide an Ethernet protocol handler for a particular protocol type
driver receives an Ethernet packet, it reads the packet header into an internal
buffer, reads the protocol type, and calls the protocol handler for that protocol
type. The CPU is in interrupt mode, and the registers are used as follows:
Registers on call to Ethernet protocol handler
this register until after the ReadRest routine has completed execution.
this register until after the ReadRest routine has completed execution.
A2 Free for your use.
A3 Pointer to first byte past data-link header bytes (the first byte after
the 2-byte protocol-type field).
A4 Pointer to the ReadPacket routine. The ReadRest routine starts 2 bytes
after the start of the ReadPacket routine.
A5 Free for your use until after the ReadRest routine has completed
execution.
D0 Free for your use.
D1 Number of bytes in the Ethernet packet left to be read (that is, the
number of bytes following the Ethernet header).
D2 Free for your use.
D3 Free for your use.
If your protocol handler processes more than one protocol type, you can read
the protocol-type field in the data-link header to determine the protocol type
of the packet. The protocol-type field starts 2 bytes before the address pointed
to by the A3 register.
Note: The source address starts 8 bytes before the address pointed to by
the A3 register, and the destination address starts 14 bytes before the
address pointed to by the A3 register.
If you know that the packet contains pad bytes and you know the actual size of
the data, you can reduce the number in the D1 register by the number of pad
bytes remaining to be read. In all other circumstances, you should not change
the value in the D1 register.
After you have called the ReadRest routine, you can use registers A0 through
A3 and D0 through D3 for your own use, but you must preserve all other
registers. You cannot depend on having access to your application global
variables.
How Your Protocol Handler Calls the .ENET Driver
ReadRest to read the incoming data packet. You may call the ReadPacket routine
as many times as you like to read the data piece by piece into one or more data
buffers, but you must always use the ReadRest routine to read the final piece of
the data packet. The ReadRest routine restores the machine state (the stack
pointers, status register, and so forth) and checks for error conditions.
Before you call the ReadPacket routine, you must allocate memory for a data
buffer and place a pointer to the buffer in the A3 register. You place the
number of bytes you want to read in the D3 register. You must not request
more bytes than remain in the data packet.
To call the ReadPacket routine, execute a JSR instruction to the address in the
A4 register. The ReadPacket routine uses the registers as follows:
Registers on entry to the ReadPacket routine
A3 Pointer to a buffer to hold the data you want to read
D3 Number of bytes to read; must be nonzero
Registers on exit from the ReadPacket routine
A0 Unchanged
A1 Unchanged
A2 Unchanged
A3 First byte after the last byte read into buffer
D0 Changed
D1 Number of bytes left to be read
D2 Unchanged
D3 Equals 0 if requested number of bytes were read, nonzero if error
The ReadPacket routine indicates an error by clearing to 0 the zero (z) flag
in the status register. If the ReadPacketroutine returns an error, you must
terminate execution of your protocol handler with an RTS instruction without
calling ReadPacket again or calling ReadRest at all.
Call the ReadRest routine to read the last portion of the data packet, or call it
after you have read all the data with ReadPacket routines and before you do any
other processing or terminate execution. You must provide in the A3 register a
pointer to a data buffer and must indicate in the D3 register the size of the data
buffer. If you have already read all of the data with calls to the ReadPacket
routine, you can specify a buffer of size 0.
Warning: If you do not call the ReadRest routine after your last call to
the ReadPacket routine, the system will crash.
To call the ReadRest routine, execute a JSR instruction to an address 2 bytes
past the address in the A4 register. The ReadRest routine uses the registers as
follows:
Registers on entry to the ReadRest routine
A3 Pointer to a buffer to hold the data you want to read
D3 Size of the buffer (short length); may be 0
Registers on exit from the ReadRest routine
A0 Unchanged
A1 Unchanged
A2 Unchanged
A3 Pointer to first byte after the last byte read into buffer
D0 Changed
D1 Changed
D2 Unchanged
D3 Equals 0 if requested number of bytes were read; less than 0 if more
data was left than would fit in buffer (extra data equals -D3 bytes);
greater than 0 if less data was left than the size of the buffer (extra
buffer space equals D3 bytes)
The ReadRest routine indicates an error by clearing to 0 the zero (z) flag in
the status register.
You must terminate execution of your protocol handler with an RTS
instruction whether or not the ReadRest routine returns an error.