nvm fixed it

src/net/torvald/tsvm/SerialHelper.kt

@@ -42,6 +42,7 @@ object SerialHelper {
     fun fetchResponse(vm: VM, portNo: Int): ByteArray {
         val incomingMsg = ByteArray(BLOCK_SIZE)
 
+        // incoming message is always 4K long and unused bytes are zero-filled. THIS IS INTENTIONAL
         UnsafeHelper.memcpyRaw(
             null, vm.getIO().blockTransferRx[portNo].ptr,
             incomingMsg, UnsafeHelper.getArrayOffset(incomingMsg),
@@ -61,9 +62,10 @@ object SerialHelper {
             waitUntilReady(vm, portNo)
 
             val transStat = getBlockTransferStatus(vm, portNo)
-            println("[SerialHelper.pullMessage()] received length: ${transStat.first}")
+            val receivedLen = transStat.first//vm.getIO().blockTransferPorts[portNo].yourBlockSize()
+            //println("[SerialHelper.pullMessage()] received length: $receivedLen")
 
-            for (k in 0 until minOf(BLOCK_SIZE, transStat.first)) {
+            for (k in 0 until minOf(BLOCK_SIZE, receivedLen)) {
                 msgBuffer.write(vm.getIO().blockTransferRx[portNo][k.toLong()].toInt())
             }
 

src/net/torvald/tsvm/peripheral/BlockTransferInterface.kt

@@ -21,15 +21,19 @@ abstract class BlockTransferInterface(val isMaster: Boolean, val isSlave: Boolea
     open fun areYouBusy(): Boolean = recipient?.busy ?: false
 
     /** Writes a thing to the recipient.
-     * A method exposed to outside of the box */
-    abstract fun startSendImpl(recipient: BlockTransferInterface)
+     * A method exposed to outside of the box
+     * @return number of bytes actually sent over*/
+    abstract fun startSendImpl(recipient: BlockTransferInterface): Int
     /** The actual implementation */
     fun startSend() {
         //if (areYouReady()) {
             busy = true
             ready = false
 
-            recipient?.let { startSendImpl(it) }
+            recipient?.let {
+                this.blockSize = startSendImpl(it)
+                //println("[BlockTransferInterface.startSend()] recipients blocksize = ${this.blockSize}")
+            }
 
             busy = false
             ready = true

src/net/torvald/tsvm/peripheral/BlockTransferPort.kt

@@ -10,8 +10,9 @@ class BlockTransferPort(val vm: VM, val portno: Int) : BlockTransferInterface(tr
 
     internal var hasNext = false
 
-    override fun startSendImpl(recipient: BlockTransferInterface) {
+    override fun startSendImpl(recipient: BlockTransferInterface): Int {
         recipient.writeout(ByteArray(BLOCK_SIZE) { vm.getIO().blockTransferTx[portno][it.toLong()] })
+        return blockSize // use MMIO to modify this variable
     }
 
     override fun hasNext(): Boolean = hasNext
@@ -22,9 +23,9 @@ class BlockTransferPort(val vm: VM, val portno: Int) : BlockTransferInterface(tr
         //UnsafeHelper.memcpyRaw(inputData, arrayOffset, null, vm.getIO().blockTransferRx[portno].ptr, copySize)
 
         // not exposing raw memory to block probable security hole
-        println("[BlockTranferPort] writeout size: ${inputData.size}")
+        //println("[BlockTranferPort] writeout size: ${inputData.size}")
         for (k in 0 until BLOCK_SIZE) {
-            vm.getIO().blockTransferRx[portno][k.toLong()] = if (k >= inputData.size) 0x5F else inputData[k]
+            vm.getIO().blockTransferRx[portno][k.toLong()] = if (k >= inputData.size) 0 else inputData[k]
         }
     }
 

src/net/torvald/tsvm/peripheral/TestDiskDrive.kt

@@ -88,7 +88,7 @@ class TestDiskDrive(private val driveNum: Int) : BlockTransferInterface(false, t
      *
      * Disk drive must send prepared message (or file transfer packet) to the computer.
      */
-    override fun startSendImpl(recipient: BlockTransferInterface) {
+    override fun startSendImpl(recipient: BlockTransferInterface): Int {
         if (blockSendCount == 0) {
             blockSendBuffer = messageComposeBuffer.toByteArray()
         }
@@ -102,6 +102,8 @@ class TestDiskDrive(private val driveNum: Int) : BlockTransferInterface(false, t
         })
 
         blockSendCount += 1
+
+        return sendSize
     }
 
     /** Computer's attempt to startSend() will result in calling this very function.

src/net/torvald/tsvm/peripheral/TestFunctionGenerator.kt

@@ -100,7 +100,7 @@ Nunc mollis nibh vitae sapien consequat, ut vestibulum sem pharetra. Aliquam iac
         return sb.toByteArray()
     }
 
-    override fun startSendImpl(recipient: BlockTransferInterface) {
+    override fun startSendImpl(recipient: BlockTransferInterface): Int {
         if (blockSendCount == 0) {
             //blockSendBuffer = messageComposeBuffer.toByteArray()
             blockSendBuffer = fileContent_multiblocks
@@ -115,6 +115,8 @@ Nunc mollis nibh vitae sapien consequat, ut vestibulum sem pharetra. Aliquam iac
         })
 
         blockSendCount += 1
+
+        return sendSize
     }
 
     override fun hasNext(): Boolean {