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comparison docs/extension-virtualports.txt @ 6:3a4c972c92ec
Add non-variable packet READ option
It is clear that variable packets are inconvenient on high speed bitbanger
implementations of the lwwire protocol. This update provides an alternative
to the variable length READ option that does not require the use of variable
length packets.
| author | William Astle <lost@l-w.ca> |
|---|---|
| date | Sat, 11 Jun 2016 11:13:49 -0600 |
| parents | bef2801ac83e |
| children |
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| 5:422f5e8fff85 | 6:3a4c972c92ec |
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| 163 less than the requested number, it means some bytes were not written for | 163 less than the requested number, it means some bytes were not written for |
| 164 whatever reason and the client must deal with queueing or buffering as | 164 whatever reason and the client must deal with queueing or buffering as |
| 165 appropriate. | 165 appropriate. |
| 166 | 166 |
| 167 | 167 |
| 168 06 VPORT_READFIXED | |
| 169 | |
| 170 This request reads bytes from a virtual port. The request looks as follows: | |
| 171 | |
| 172 Octet Meaning | |
| 173 ----- ------- | |
| 174 0-2 F3 00 04 | |
| 175 3 port number | |
| 176 4 size to read in octets (N) | |
| 177 | |
| 178 The response is looks as follows: | |
| 179 | |
| 180 Octet Meaning | |
| 181 ----- ------- | |
| 182 0 status | |
| 183 1 number of bytes returned (n) | |
| 184 2-... returned data | |
| 185 | |
| 186 In all cases, there will be exactly N octets in the returned data. Any bytes | |
| 187 not filled by actual data are undefined but should be zero. | |
| 188 | |
| 189 If status is nonzero, then the read failed. This will usually be due to a | |
| 190 lack of data (1) OR the port not being open (2). In this case, any | |
| 191 subsequent octets in the packet can be ignored. | |
| 192 | |
| 193 If (n) is less than (N), then there will be extra octets in the returned | |
| 194 data section which must be ignored. | |
| 195 | |
| 196 This option is provided as an alternative to the VPORT_READ call for systems | |
| 197 which cannot easily handle a variable length packet safely. In particular, a | |
| 198 115k "bitbanger" implementation on the Coco itself might choose to use this | |
| 199 option to adding another 500+ bytes to its lwwire client driver. Ohter | |
| 200 communication channels that do not have this limitation should prefer the | |
| 201 variable length packet option with larger request sizes to minimize | |
| 202 transaction count. | |
| 203 |