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+</head>
+<body>
+<div id="content" class="content">
+<h1 class="title">LPT Communication Driver</h1>
+<div id="table-of-contents" role="doc-toc">
+<h2>Table of Contents</h2>
+<div id="text-table-of-contents" role="doc-toc">
+<ul>
+<li><a href="#orga5cdfb9">1. Overview</a></li>
+<li><a href="#org933ae73">2. LPT Communication Driver</a>
+<ul>
+<li><a href="#org193f48a">2.1. Overview</a></li>
+<li><a href="#orge83c8c0">2.2. Data transmission implementation details</a></li>
+<li><a href="#org096cdc9">2.3. Driver API</a>
+<ul>
+<li><a href="#orgcf7457f">2.3.1. Deactivate the driver</a></li>
+<li><a href="#orgb25a6e0">2.3.2. Activate the driver</a></li>
+<li><a href="#org50591bb">2.3.3. Retrieve downloaded data from the driver's input buffer</a></li>
+<li><a href="#org0867f81">2.3.4. Upload data to the driver's output buffer for transmission</a></li>
+</ul>
+</li>
+</ul>
+</li>
+</ul>
+</div>
+</div>
+
+<div id="outline-container-orga5cdfb9" class="outline-2">
+<h2 id="orga5cdfb9"><span class="section-number-2">1.</span> Overview</h2>
+<div class="outline-text-2" id="text-1">
+<p>
+This is weird networking solution. It allows to send data using
+parallel LPT port serially(!) by bit-banging between two connected
+computers :)
+</p>
+
+<p>
+Out of <a href="https://en.wikipedia.org/wiki/Parallel_port">25 physical wires in LPT port</a>, only 3 are used:
+</p>
+
+<dl class="org-dl">
+<dt>Pin 14</dt><dd>Carries a synchronization signal which uses a periodic
+pattern (e.g., 010101…) to maintain timing alignment between the
+communicating computers.</dd>
+
+<dt>Pin 17</dt><dd>Functions as the bidirectional data line, responsible for
+transmitting and receiving data between the connected computers.</dd>
+
+<dt>Pin 18</dt><dd>Acts as the ground connection, providing a common
+reference for electrical signals to ensure consistency in
+communication.</dd>
+</dl>
+
+
+<div id="org3425788" class="figure">
+<p><img src="diagram.png" alt="diagram.png" />
+</p>
+</div>
+
+<p>
+By utilizing only three wires and software controlled bit-banging
+algorithm, custom, comparatively simple, cheap and long cable can be
+built to connect 2 computers in a DIY network setup.
+</p>
+</div>
+</div>
+
+<div id="outline-container-org933ae73" class="outline-2">
+<h2 id="org933ae73"><span class="section-number-2">2.</span> LPT Communication Driver</h2>
+<div class="outline-text-2" id="text-2">
+</div>
+<div id="outline-container-org193f48a" class="outline-3">
+<h3 id="org193f48a"><span class="section-number-3">2.1.</span> Overview</h3>
+<div class="outline-text-3" id="text-2-1">
+<p>
+The LPT Communication Driver is a Terminate and Stay Resident (TSR)
+driver designed to facilitate communication between computers using
+parallel printer ports (LPT). This driver uses bit-banging to send and
+receive data serially over the LPT port, utilizing only three wires
+for communication.
+</p>
+
+<p>
+Driver hooks into the system's IRQ 0 to ensure that system timer
+always keeps executing it in the background. While operating as a
+background process, it periodically monitors LPT port to detect
+incoming transmission. When transmission is detected, driver receives,
+decodes and stores it into preallocated 5000 byte receive buffer.
+</p>
+
+<p>
+Applications can then communicate with the driver on their own
+schedule using INT 63h to poll for and retrieve received messages, if
+any. Applications can also send outgoing messages into 5000 byte
+driver outbox memory buffer. Thereafter driver will transmit those
+messages in background mode over the wire.
+</p>
+
+<p>
+The driver is half-duplex: it prioritizes receiving over sending and
+does not transmit while receiving.
+</p>
+
+<p>
+During active transmission/reception, the driver can consume 100% CPU
+due to busy-wait loops in the IRQ handler, potentially causing random
+temporary hiccups for application running in the
+foreground. Unsuitable for real-time systems.
+</p>
+
+<p>
+Download:
+</p>
+<ul class="org-ul">
+<li>Source code: <a href="lptdrv.asm">lptdrv.asm</a></li>
+<li>Compiled binary: <a href="lptdrv.com">lptdrv.com</a></li>
+</ul>
+</div>
+</div>
+
+<div id="outline-container-orge83c8c0" class="outline-3">
+<h3 id="orge83c8c0"><span class="section-number-3">2.2.</span> Data transmission implementation details</h3>
+<div class="outline-text-3" id="text-2-2">
+<p>
+When there is incoming data transmission, the TSR driver detects it
+during its periodic execution in the IRQ 0 (timer) handler, which runs
+approximately every 55ms.
+</p>
+
+<p>
+Driver checks for possible transmission comparatively rarely (every 55
+ms) and for this reason it is important to have quite long
+transmission start indicator/header before actual data is sent. This
+allows long enough time for recipient computer communication driver to
+detect that transmission line is active and start listening to it now
+in exclusive busy-wait loop. So, the TSR driver steals 100% of the CPU
+for the duration of transmission.
+</p>
+
+<p>
+The start of transmission is detected by reading the port (37Ah)
+value, and checking if bit 3 of the raw input is high. It then enters
+a "skip header" loop. Once bit 1 goes low, bit reception begins. The
+end is detected via a timeout: during bit reception, a counter
+increments on each poll. If there is no change in the port value for
+30 consecutive polls (indicating no new bit transition), it assumes
+the transmission is complete, appends a 2-byte length to the receive
+buffer, and exits the routine.
+</p>
+
+<p>
+So, both receive and send routines execute within the IRQ 0 handler
+using busy-wait polling loops (for receive) or timed output loops (for
+send). These can hold the CPU for the full duration of a transmission,
+as the handler does not yield until complete.
+</p>
+
+<p>
+It bit-bangs data by treating the LPT control port (37Ah) as both
+output and input, using bit 3 (pin 17, data line) for the serial data
+bit and bit 1 (pin 14, sync line) for a clock that alternates (low on
+even bit indices, high on odd) to signal transitions.
+</p>
+
+<p>
+For sending: the port is first set to 0xFF (all bits high) as a
+header, held for ~110ms (2 timer ticks).
+</p>
+
+<p>
+For receiving: after start detection, it polls the port for value
+changes (transitions). On each change, it reads the port again (to
+ensure that synchronization bit did not arrive ahead data bit over the
+separate physical wire), extracts bit 3 as the data bit, shifts it
+into a byte accumulator, and resets the timeout counter. Once a full
+byte is accumulated, it stores it in the receive buffer. No ACK or
+error checking.
+</p>
+
+<p>
+Driver can receive multiple transmissions into its 5000-byte receive
+buffer before the client program reads it out. Each incoming
+transmission appends its data bytes followed by a 2-byte length word
+directly to the end of the buffer (updating dbufsiz to the new total
+used). As long as the cumulative size doesn't exceed 5000 bytes,
+multiple can queue up. The buffer acts as a FIFO for concatenated
+packets; overflows are not handled (it would corrupt without
+checks). The client retrieves the entire buffer contents at once when
+polling.
+</p>
+
+<p>
+When the client program reads received data from the TSR (via INT 63h
+AH=2), the driver copies the full buffer contents (up to dbufsiz
+bytes) to the client's specified ES:DI pointer, returns the byte count
+in AX, and immediately resets dbufsiz to 0, clearing the buffer. This
+ensures the data is not served again on subsequent reads, as the
+buffer is emptied after each retrieval. If no data is available, AX=0
+is returned.
+</p>
+</div>
+</div>
+
+<div id="outline-container-org096cdc9" class="outline-3">
+<h3 id="org096cdc9"><span class="section-number-3">2.3.</span> Driver API</h3>
+<div class="outline-text-3" id="text-2-3">
+<p>
+The driver uses INT 63h for its API, with functions selected via the
+AH register. It maintains two internal buffers:
+</p>
+
+<ul class="org-ul">
+<li>Download Buffer: 5000 bytes for incoming (received) data. Multiple
+transmissions can be queued here, each appended with a 2-byte length
+footer.</li>
+<li>Upload Buffer: 5000 bytes for outgoing (to-be-sent) data. Data is
+copied here and transmitted when the line is free.</li>
+</ul>
+
+
+<p>
+Communication is polling-based for applications; the driver handles
+transmission/reception in the background.
+</p>
+
+<p>
+No error checking, acknowledgments, or flow control; it's a simple,
+unidirectional-per-turn protocol.
+</p>
+
+<p>
+To use the driver:
+</p>
+<ul class="org-ul">
+<li>Load the TSR (e.g., run lptdrv.com).</li>
+<li>Activate it via the API.</li>
+<li>Poll for received data or queue sends as needed.</li>
+<li>Deactivate when done.</li>
+</ul>
+
+<p>
+API overview:
+</p>
+<table border="2" cellspacing="0" cellpadding="6" rules="groups" frame="hsides">
+
+
+<colgroup>
+<col class="org-right" />
+
+<col class="org-left" />
+</colgroup>
+<thead>
+<tr>
+<th scope="col" class="org-right">AH register</th>
+<th scope="col" class="org-left">API function</th>
+</tr>
+</thead>
+<tbody>
+<tr>
+<td class="org-right">0</td>
+<td class="org-left"><a href="#orgcf7457f">Deactivate the driver</a></td>
+</tr>
+
+<tr>
+<td class="org-right">1</td>
+<td class="org-left"><a href="#orgb25a6e0">Activate the driver</a></td>
+</tr>
+
+<tr>
+<td class="org-right">2</td>
+<td class="org-left"><a href="#org50591bb">Retrieve downloaded data from the driver's input buffer</a></td>
+</tr>
+
+<tr>
+<td class="org-right">3</td>
+<td class="org-left"><a href="#org0867f81">Upload data to the driver's output buffer for transmission</a></td>
+</tr>
+</tbody>
+</table>
+</div>
+
+<div id="outline-container-orgcf7457f" class="outline-4">
+<h4 id="orgcf7457f"><span class="section-number-4">2.3.1.</span> Deactivate the driver</h4>
+<div class="outline-text-4" id="text-2-3-1">
+<p>
+Disables the driver, stopping background monitoring and
+transmission. The LPT port is reset (set to 0).
+</p>
+
+<ul class="org-ul">
+<li>AH: 0</li>
+<li>Parameters: None</li>
+<li>Returns: None</li>
+<li>Side Effects: Clears the enabled flag.</li>
+<li>Usage Notes: Call this before unloading the TSR or when
+communication is no longer needed to free system resources.</li>
+</ul>
+</div>
+</div>
+
+<div id="outline-container-orgb25a6e0" class="outline-4">
+<h4 id="orgb25a6e0"><span class="section-number-4">2.3.2.</span> Activate the driver</h4>
+<div class="outline-text-4" id="text-2-3-2">
+<p>
+Enables the driver, starting background LPT port monitoring for
+incoming data. The LPT port is reset (set to 0) upon activation.
+</p>
+
+<ul class="org-ul">
+<li>AH: 1</li>
+<li>Parameters: None</li>
+<li>Returns: None</li>
+<li>Side Effects: Sets the enabled flag. Existing buffer contents are
+preserved.</li>
+<li>Usage Notes: Must be called after loading the TSR and before any
+send/receive operations. Can be called multiple times; redundant
+activations are harmless.</li>
+</ul>
+</div>
+</div>
+
+<div id="outline-container-org50591bb" class="outline-4">
+<h4 id="org50591bb"><span class="section-number-4">2.3.3.</span> Retrieve downloaded data from the driver's input buffer</h4>
+<div class="outline-text-4" id="text-2-3-3">
+<p>
+Copies all accumulated received data from the driver's download buffer
+to the caller's memory location and clears the buffer.
+</p>
+
+<ul class="org-ul">
+<li><b>AH</b> : 2</li>
+<li><b>Parameters</b> :
+<ul class="org-ul">
+<li><b>ES:DI</b> : Pointer to the buffer where received data should be
+copied (must be large enough to hold up to 5000 bytes).</li>
+</ul></li>
+<li><b>Returns</b> :
+<ul class="org-ul">
+<li><b>AX</b> : Number of bytes copied (0 if no data available).</li>
+</ul></li>
+<li><b>Side Effects</b> : Resets the download buffer size to 0, preventing
+re-retrieval of the same data.</li>
+<li><b>Usage Notes</b> :
+<ul class="org-ul">
+<li>Data is retrieved as a concatenated stream of all queued
+transmissions.</li>
+<li>Each transmission in the buffer ends with a 2-byte length word
+(little-endian) indicating its payload size (excluding the length
+itself).</li>
+<li>Poll this function periodically in a loop to check for new data.</li>
+<li>If AX=0, no copy occurs.</li>
+<li>Example: In assembly, set ES:DI to your receive buffer and call
+INT 63h; then process AX bytes if >0.</li>
+</ul></li>
+</ul>
+</div>
+</div>
+
+<div id="outline-container-org0867f81" class="outline-4">
+<h4 id="org0867f81"><span class="section-number-4">2.3.4.</span> Upload data to the driver's output buffer for transmission</h4>
+<div class="outline-text-4" id="text-2-3-4">
+<p>
+Copies the specified data to the driver's upload buffer for background
+transmission. Transmission occurs when the line is free (no incoming
+data).
+</p>
+
+<ul class="org-ul">
+<li><b>AH</b> : 3</li>
+<li><b>Parameters</b> :
+<ul class="org-ul">
+<li><b>DS:SI</b> : Pointer to the data to upload.</li>
+<li><b>CX</b> : Number of bytes to upload (must not exceed remaining upload
+buffer space; no checks performed).</li>
+</ul></li>
+<li><b>Returns</b> : None</li>
+<li><b>Side Effects</b> : Appends data to the upload buffer and updates its
+size. Transmission is asynchronous.</li>
+<li><b>Usage Notes</b> :
+<ul class="org-ul">
+<li>Data is sent as a single transmission (no automatic framing;
+caller can add headers if needed).</li>
+<li>If the buffer is full (total >5000 bytes), behavior is undefined
+(overflow).</li>
+<li>Multiple calls can queue data sequentially in the buffer.</li>
+<li>Transmission starts in the next IRQ 0 tick if the line is idle.</li>
+<li>The driver adds no footer; the receiver sees exactly the sent bytes.</li>
+<li>Example: Load DS:SI with your message, CX with length, call INT
+63h; the driver handles sending.</li>
+</ul></li>
+</ul>
+</div>
+</div>
+</div>
+</div>
+</div>
+<div id="postamble" class="status">
+<p class="date">Created: 2025-08-04 ma 02:12</p>
+<p class="validation"><a href="https://validator.w3.org/check?uri=referer">Validate</a></p>
+</div>
+</body>
+</html>
reference for electrical signals to ensure consistency in\r
communication.\r
\r
+[[file:diagram.png]]\r
\r
By utilizing only three wires and software controlled bit-banging\r
algorithm, custom, comparatively simple, cheap and long cable can be\r
\r
* LPT Communication Driver\r
\r
+** Overview\r
\r
The LPT Communication Driver is a Terminate and Stay Resident (TSR)\r
driver designed to facilitate communication between computers using\r
driver outbox memory buffer. Thereafter driver will transmit those\r
messages in background mode over the wire.\r
\r
+The driver is half-duplex: it prioritizes receiving over sending and\r
+does not transmit while receiving.\r
+\r
+During active transmission/reception, the driver can consume 100% CPU\r
+due to busy-wait loops in the IRQ handler, potentially causing random\r
+temporary hiccups for application running in the\r
+foreground. Unsuitable for real-time systems.\r
+\r
Download:\r
- Source code: [[file:lptdrv.asm][lptdrv.asm]]\r
- Compiled binary: [[file:lptdrv.com][lptdrv.com]]\r
\r
+** Data transmission implementation details\r
+\r
+When there is incoming data transmission, the TSR driver detects it\r
+during its periodic execution in the IRQ 0 (timer) handler, which runs\r
+approximately every 55ms.\r
+\r
+Driver checks for possible transmission comparatively rarely (every 55\r
+ms) and for this reason it is important to have quite long\r
+transmission start indicator/header before actual data is sent. This\r
+allows long enough time for recipient computer communication driver to\r
+detect that transmission line is active and start listening to it now\r
+in exclusive busy-wait loop. So, the TSR driver steals 100% of the CPU\r
+for the duration of transmission.\r
+\r
+The start of transmission is detected by reading the port (37Ah)\r
+value, and checking if bit 3 of the raw input is high. It then enters\r
+a "skip header" loop. Once bit 1 goes low, bit reception begins. The\r
+end is detected via a timeout: during bit reception, a counter\r
+increments on each poll. If there is no change in the port value for\r
+30 consecutive polls (indicating no new bit transition), it assumes\r
+the transmission is complete, appends a 2-byte length to the receive\r
+buffer, and exits the routine.\r
+\r
+So, both receive and send routines execute within the IRQ 0 handler\r
+using busy-wait polling loops (for receive) or timed output loops (for\r
+send). These can hold the CPU for the full duration of a transmission,\r
+as the handler does not yield until complete.\r
+\r
+It bit-bangs data by treating the LPT control port (37Ah) as both\r
+output and input, using bit 3 (pin 17, data line) for the serial data\r
+bit and bit 1 (pin 14, sync line) for a clock that alternates (low on\r
+even bit indices, high on odd) to signal transitions.\r
+\r
+For sending: the port is first set to 0xFF (all bits high) as a\r
+header, held for ~110ms (2 timer ticks).\r
+\r
+For receiving: after start detection, it polls the port for value\r
+changes (transitions). On each change, it reads the port again (to\r
+ensure that synchronization bit did not arrive ahead data bit over the\r
+separate physical wire), extracts bit 3 as the data bit, shifts it\r
+into a byte accumulator, and resets the timeout counter. Once a full\r
+byte is accumulated, it stores it in the receive buffer. No ACK or\r
+error checking.\r
+\r
+Driver can receive multiple transmissions into its 5000-byte receive\r
+buffer before the client program reads it out. Each incoming\r
+transmission appends its data bytes followed by a 2-byte length word\r
+directly to the end of the buffer (updating dbufsiz to the new total\r
+used). As long as the cumulative size doesn't exceed 5000 bytes,\r
+multiple can queue up. The buffer acts as a FIFO for concatenated\r
+packets; overflows are not handled (it would corrupt without\r
+checks). The client retrieves the entire buffer contents at once when\r
+polling.\r
+\r
+When the client program reads received data from the TSR (via INT 63h\r
+AH=2), the driver copies the full buffer contents (up to dbufsiz\r
+bytes) to the client's specified ES:DI pointer, returns the byte count\r
+in AX, and immediately resets dbufsiz to 0, clearing the buffer. This\r
+ensures the data is not served again on subsequent reads, as the\r
+buffer is emptied after each retrieval. If no data is available, AX=0\r
+is returned.\r
\r
** Driver API\r
-*** Deactivate the driver\r
\r
-*Parameters*:\r
-: AH = 0\r
+The driver uses INT 63h for its API, with functions selected via the\r
+AH register. It maintains two internal buffers:\r
\r
-*Returns*: None\r
+- Download Buffer: 5000 bytes for incoming (received) data. Multiple\r
+ transmissions can be queued here, each appended with a 2-byte length\r
+ footer.\r
+- Upload Buffer: 5000 bytes for outgoing (to-be-sent) data. Data is\r
+ copied here and transmitted when the line is free.\r
\r
-*** Activates the driver\r
\r
-*Parameters*:\r
-: AH = 1\r
+Communication is polling-based for applications; the driver handles\r
+transmission/reception in the background.\r
\r
-*Returns*: None\r
-\r
-*** Retrieve downloaded data from the driver's input buffer\r
+No error checking, acknowledgments, or flow control; it's a simple,\r
+unidirectional-per-turn protocol.\r
\r
-*Parameters*:\r
-: AH = 2\r
-: ES:DI = Pointer to the location where downloaded data should be placed\r
+To use the driver:\r
+- Load the TSR (e.g., run lptdrv.com).\r
+- Activate it via the API.\r
+- Poll for received data or queue sends as needed.\r
+- Deactivate when done.\r
\r
-*Returns*:\r
-: AX : Number of bytes downloaded\r
+API overview:\r
+| AH register | API function |\r
+|-------------+---------------------------------------------------------|\r
+| 0 | [[id:a7aaa0e6-92de-467c-bcd4-b3d3216b15d4][Deactivate the driver]] |\r
+| 1 | [[id:944be7b6-d3ba-486a-98bd-1a66cfffe6e5][Activate the driver]] |\r
+| 2 | [[id:49350196-55b9-4d50-b672-b3c6d6d55e53][Retrieve downloaded data from the driver's input buffer]] |\r
+| 3 | [[id:53fd0c68-4057-4e9e-b908-87fab6eab5c8][Upload data to the driver's output buffer for transmission]] |\r
\r
-*** Upload data to the driver's output buffer for transmission\r
+*** Deactivate the driver\r
+:PROPERTIES:\r
+:ID: a7aaa0e6-92de-467c-bcd4-b3d3216b15d4\r
+:END:\r
+\r
+Disables the driver, stopping background monitoring and\r
+transmission. The LPT port is reset (set to 0).\r
+\r
+- AH: 0\r
+- Parameters: None\r
+- Returns: None\r
+- Side Effects: Clears the enabled flag.\r
+- Usage Notes: Call this before unloading the TSR or when\r
+ communication is no longer needed to free system resources.\r
+\r
+*** Activate the driver\r
+:PROPERTIES:\r
+:ID: 944be7b6-d3ba-486a-98bd-1a66cfffe6e5\r
+:END:\r
+\r
+Enables the driver, starting background LPT port monitoring for\r
+incoming data. The LPT port is reset (set to 0) upon activation.\r
+\r
+- AH: 1\r
+- Parameters: None\r
+- Returns: None\r
+- Side Effects: Sets the enabled flag. Existing buffer contents are\r
+ preserved.\r
+- Usage Notes: Must be called after loading the TSR and before any\r
+ send/receive operations. Can be called multiple times; redundant\r
+ activations are harmless.\r
\r
-*Parameters*:\r
-: AH = 3\r
-: DS:SI: Pointer to the data to be uploaded\r
-: CX: Number of bytes to upload\r
+*** Retrieve downloaded data from the driver's input buffer\r
+:PROPERTIES:\r
+:ID: 49350196-55b9-4d50-b672-b3c6d6d55e53\r
+:END:\r
+\r
+Copies all accumulated received data from the driver's download buffer\r
+to the caller's memory location and clears the buffer.\r
+\r
+- *AH* : 2\r
+- *Parameters* :\r
+ - *ES:DI* : Pointer to the buffer where received data should be\r
+ copied (must be large enough to hold up to 5000 bytes).\r
+- *Returns* :\r
+ - *AX* : Number of bytes copied (0 if no data available).\r
+- *Side Effects* : Resets the download buffer size to 0, preventing\r
+ re-retrieval of the same data.\r
+- *Usage Notes* :\r
+ - Data is retrieved as a concatenated stream of all queued\r
+ transmissions.\r
+ - Each transmission in the buffer ends with a 2-byte length word\r
+ (little-endian) indicating its payload size (excluding the length\r
+ itself).\r
+ - Poll this function periodically in a loop to check for new data.\r
+ - If AX=0, no copy occurs.\r
+ - Example: In assembly, set ES:DI to your receive buffer and call\r
+ INT 63h; then process AX bytes if >0.\r
\r
-*Returns*: None\r
+*** Upload data to the driver's output buffer for transmission\r
+:PROPERTIES:\r
+:ID: 53fd0c68-4057-4e9e-b908-87fab6eab5c8\r
+:END:\r
+\r
+Copies the specified data to the driver's upload buffer for background\r
+transmission. Transmission occurs when the line is free (no incoming\r
+data).\r
+\r
+- *AH* : 3\r
+- *Parameters* :\r
+ - *DS:SI* : Pointer to the data to upload.\r
+ - *CX* : Number of bytes to upload (must not exceed remaining upload\r
+ buffer space; no checks performed).\r
+- *Returns* : None\r
+- *Side Effects* : Appends data to the upload buffer and updates its\r
+ size. Transmission is asynchronous.\r
+- *Usage Notes* :\r
+ - Data is sent as a single transmission (no automatic framing;\r
+ caller can add headers if needed).\r
+ - If the buffer is full (total >5000 bytes), behavior is undefined\r
+ (overflow).\r
+ - Multiple calls can queue data sequentially in the buffer.\r
+ - Transmission starts in the next IRQ 0 tick if the line is idle.\r
+ - The driver adds no footer; the receiver sees exactly the sent bytes.\r
+ - Example: Load DS:SI with your message, CX with length, call INT\r
+ 63h; the driver handles sending.\r
projects / qbasicapps.git / commitdiff
