WinComm/drv/isr.c

// isr.c - Interrupt service routines for COMM.DRV replacement
//
// ISR entry points for IRQ3 (COM2/4) and IRQ4 (COM1/3), DPMI interrupt
// vector hooking/unhooking, and interrupt dispatch with 16550 FIFO support.

#include "commdrv.h"
#include <dos.h>

// -----------------------------------------------------------------------
// Prototypes
// -----------------------------------------------------------------------
static void checkFlowRx(PortStateT *port);
static void checkNotify(PortStateT *port);
static void handleLsr(PortStateT *port, uint8_t lsr);
static void handleMsr(PortStateT *port);
static void handleRx(PortStateT *port, uint8_t lsr);
static void handleTx(PortStateT *port);

void _far _interrupt isr3(void);
void _far _interrupt isr4(void);

// -----------------------------------------------------------------------
// Saved previous ISR vectors for IRQ3 and IRQ4
// -----------------------------------------------------------------------
static void (_far _interrupt *prevIsr3)(void) = NULL;
static void (_far _interrupt *prevIsr4)(void) = NULL;

// Track how many ports are using each IRQ so we know when to unhook
static int16_t irq3RefCount = 0;
static int16_t irq4RefCount = 0;


// -----------------------------------------------------------------------
// checkFlowRx - Check RX buffer level and assert/deassert flow control
// -----------------------------------------------------------------------
static void checkFlowRx(PortStateT *port)
{
    uint16_t base;

    base = port->baseAddr;

    if (port->hsMode == HS_NONE) {
        return;
    }

    // Check if we need to stop remote sender
    if (!port->rxStopped && port->rxCount >= (port->rxSize - port->xoffLim)) {
        port->rxStopped = TRUE;

        if (port->hsMode == HS_XONXOFF || port->hsMode == HS_BOTH) {
            // Send XOFF - queue as immediate character
            port->txImmediate = port->xoffChar;
            // Enable THRE to get it sent
            _outp(base + UART_IER, (uint8_t)(_inp(base + UART_IER) | IER_THRE));
        }
        if (port->hsMode == HS_RTSCTS || port->hsMode == HS_BOTH) {
            // Drop RTS
            _outp(base + UART_MCR, (uint8_t)(_inp(base + UART_MCR) & ~MCR_RTS));
        }
    }

    // Check if we can resume remote sender
    if (port->rxStopped && port->rxCount <= port->xonLim) {
        port->rxStopped = FALSE;

        if (port->hsMode == HS_XONXOFF || port->hsMode == HS_BOTH) {
            // Send XON
            port->txImmediate = port->xonChar;
            _outp(base + UART_IER, (uint8_t)(_inp(base + UART_IER) | IER_THRE));
        }
        if (port->hsMode == HS_RTSCTS || port->hsMode == HS_BOTH) {
            // Raise RTS
            _outp(base + UART_MCR, (uint8_t)(_inp(base + UART_MCR) | MCR_RTS));
        }
    }
}


// -----------------------------------------------------------------------
// checkNotify - Post WM_COMMNOTIFY if threshold conditions met
//
// Called at end of ISR dispatch, outside the IIR loop.
// Uses PostMessage to avoid reentrancy issues.
// -----------------------------------------------------------------------
static void checkNotify(PortStateT *port)
{
    uint16_t notifyBits;

    if (!port->hwndNotify) {
        return;
    }

    notifyBits = 0;

    // CN_RECEIVE: rxCount crossed threshold from below
    if (port->rxNotifyThresh >= 0 && port->rxCount >= (uint16_t)port->rxNotifyThresh) {
        notifyBits |= CN_RECEIVE;
    }

    // CN_TRANSMIT: space available crossed threshold
    if (port->txNotifyThresh >= 0) {
        uint16_t txFree = port->txSize - port->txCount;
        if (txFree >= (uint16_t)port->txNotifyThresh) {
            notifyBits |= CN_TRANSMIT;
        }
    }

    // CN_EVENT: any event bits accumulated
    if (port->comDeb.evtWord & port->comDeb.evtMask) {
        notifyBits |= CN_EVENT;
    }

    if (notifyBits && pfnPostMessage) {
        pfnPostMessage(port->hwndNotify, WM_COMMNOTIFY, (WPARAM)port->commId, (LPARAM)notifyBits);
    }
}


// -----------------------------------------------------------------------
// handleLsr - Process line status errors
// -----------------------------------------------------------------------
static void handleLsr(PortStateT *port, uint8_t lsr)
{
    if (lsr & LSR_OE) {
        port->errorFlags |= CE_OVERRUN;
    }
    if (lsr & LSR_PE) {
        port->errorFlags |= CE_RXPARITY;
    }
    if (lsr & LSR_FE) {
        port->errorFlags |= CE_FRAME;
    }
    if (lsr & LSR_BI) {
        port->errorFlags |= CE_BREAK;
        port->comDeb.evtWord |= EV_BREAK;
    }
    if (lsr & (LSR_OE | LSR_PE | LSR_FE)) {
        port->comDeb.evtWord |= EV_ERR;
    }
}


// -----------------------------------------------------------------------
// handleMsr - Process modem status changes
// -----------------------------------------------------------------------
static void handleMsr(PortStateT *port)
{
    uint8_t msr;

    msr = (uint8_t)_inp(port->baseAddr + UART_MSR);

    // Update ComDEB MSR shadow (offset 35) for third-party code
    port->comDeb.msrShadow = msr;

    if (msr & MSR_DCTS) {
        port->comDeb.evtWord |= EV_CTS;

        // RTS/CTS flow control: stop/start TX based on CTS
        if (port->hsMode == HS_RTSCTS || port->hsMode == HS_BOTH) {
            if (msr & MSR_CTS) {
                port->txStopped = FALSE;
                // Resume TX
                if (port->txCount > 0) {
                    handleTx(port);
                }
            } else {
                port->txStopped = TRUE;
            }
        }
    }

    if (msr & MSR_DDSR) {
        port->comDeb.evtWord |= EV_DSR;
    }

    if (msr & MSR_TERI) {
        port->comDeb.evtWord |= EV_RING;
    }

    if (msr & MSR_DDCD) {
        port->comDeb.evtWord |= EV_RLSD;
    }
}


// -----------------------------------------------------------------------
// handleRx - Read all available bytes from UART into RX ring buffer
// -----------------------------------------------------------------------
static void handleRx(PortStateT *port, uint8_t lsr)
{
    uint16_t base;
    uint8_t  ch;

    base = port->baseAddr;

    while (lsr & LSR_DR) {
        // Check for line errors on this byte
        if (lsr & (LSR_OE | LSR_PE | LSR_FE | LSR_BI)) {
            handleLsr(port, lsr);
        }

        ch = (uint8_t)_inp(base + UART_RBR);

        // Check for XON/XOFF if software flow control enabled
        if (port->hsMode == HS_XONXOFF || port->hsMode == HS_BOTH) {
            if (ch == port->xonChar) {
                port->txStopped = FALSE;
                // Resume TX if we have data
                if (port->txCount > 0) {
                    handleTx(port);
                }
                lsr = (uint8_t)_inp(base + UART_LSR);
                continue;
            }
            if (ch == port->xoffChar) {
                port->txStopped = TRUE;
                lsr = (uint8_t)_inp(base + UART_LSR);
                continue;
            }
        }

        // Store in ring buffer
        if (port->rxCount < port->rxSize) {
            port->rxBuf[port->rxHead] = ch;
            port->rxHead++;
            if (port->rxHead >= port->rxSize) {
                port->rxHead = 0;
            }
            port->rxCount++;
        } else {
            // Buffer overflow
            port->errorFlags |= CE_RXOVER;
        }

        // Set event bit
        port->comDeb.evtWord |= EV_RXCHAR;

        // Read LSR again for next byte
        lsr = (uint8_t)_inp(base + UART_LSR);
    }

    // Check if we need to assert flow control
    checkFlowRx(port);
}


// -----------------------------------------------------------------------
// handleTx - Transmit bytes from TX ring buffer to UART
//
// For 16550: burst up to 16 bytes per THRE interrupt.
// For 8250: send 1 byte at a time.
// -----------------------------------------------------------------------
static void handleTx(PortStateT *port)
{
    uint16_t base;
    uint16_t burst;
    uint16_t i;

    if (port->txStopped) {
        return;
    }

    base = port->baseAddr;

    // Priority character first
    if (port->txImmediate >= 0) {
        _outp(base + UART_THR, (uint8_t)port->txImmediate);
        port->txImmediate = -1;
        return;
    }

    if (port->txCount == 0) {
        // Nothing to send -- disable THRE interrupt
        _outp(base + UART_IER, (uint8_t)(_inp(base + UART_IER) & ~IER_THRE));
        // TX empty event
        port->comDeb.evtWord |= EV_TXEMPTY;
        return;
    }

    // Burst size: 16 for FIFO, 1 for non-FIFO or FIFO disabled
    burst = (port->is16550 && port->fifoEnabled) ? FIFO_DEPTH : 1;
    if (burst > port->txCount) {
        burst = port->txCount;
    }

    for (i = 0; i < burst; i++) {
        _outp(base + UART_THR, port->txBuf[port->txTail]);
        port->txTail++;
        if (port->txTail >= port->txSize) {
            port->txTail = 0;
        }
        port->txCount--;
    }
}


// -----------------------------------------------------------------------
// hookIsr - Hook the interrupt vector for a port's IRQ via DPMI
//
// Uses INT 31h AX=0204h to get and AX=0205h to set protected-mode
// interrupt vectors.
//
// Returns 0 on success, -1 on error.
// -----------------------------------------------------------------------
int16_t hookIsr(PortStateT *port)
{
    uint8_t  intNum;
    uint8_t  picMask;
    int16_t *refCount;
    void (_far _interrupt **prevPtr)(void);
    void (_far _interrupt *newIsr)(void);
    void _far *oldVector;
    uint16_t oldSeg;
    uint16_t oldOff;

    intNum = port->irq + 8;
    port->irqMask = (uint8_t)(1 << port->irq);

    if (port->irq == 3) {
        prevPtr  = &prevIsr3;
        refCount = &irq3RefCount;
        newIsr   = isr3;
    } else {
        prevPtr  = &prevIsr4;
        refCount = &irq4RefCount;
        newIsr   = isr4;
    }

    // Only hook the vector on first use of this IRQ
    if (*refCount == 0) {
        // INT 31h AX=0204h: Get Protected Mode Interrupt Vector
        // BL = interrupt number
        // Returns: CX:DX = selector:offset of handler
        _asm {
            mov  ax, 0204h
            mov  bl, intNum
            int  31h
            mov  oldSeg, cx
            mov  oldOff, dx
        }

        oldVector = (void _far *)((uint32_t)oldSeg << 16 | oldOff);
        *prevPtr = (void (_far _interrupt *)(void))oldVector;

        // INT 31h AX=0205h: Set Protected Mode Interrupt Vector
        // BL = interrupt number
        // CX:DX = selector:offset of new handler
        {
            uint16_t newSeg = _FP_SEG(newIsr);
            uint16_t newOff = _FP_OFF(newIsr);

            _asm {
                mov  ax, 0205h
                mov  bl, intNum
                mov  cx, newSeg
                mov  dx, newOff
                int  31h
            }
        }
    }

    (*refCount)++;
    port->prevIsr = (void (FAR *)(void))*prevPtr;

    // Unmask IRQ at PIC
    picMask = _inp(PIC_DATA);
    picMask &= ~port->irqMask;
    _outp(PIC_DATA, picMask);

    return 0;
}


// -----------------------------------------------------------------------
// isrDispatch - Main interrupt dispatch loop
//
// Called from the ISR entry points with a pointer to the port state.
// Reads IIR in a loop until no more interrupts are pending.
// Handles in priority order: LSR > RX > TX > MSR.
// -----------------------------------------------------------------------
void isrDispatch(PortStateT *port)
{
    uint8_t  iir;
    uint8_t  lsr;
    uint16_t base;

    base = port->baseAddr;

    for (;;) {
        iir = (uint8_t)_inp(base + UART_IIR);

        // Bit 0 set = no interrupt pending
        if (iir & IIR_PENDING) {
            break;
        }

        switch (iir & IIR_ID_MASK) {
            case IIR_LSR:
                // Line status: read LSR, accumulate errors
                lsr = (uint8_t)_inp(base + UART_LSR);
                handleLsr(port, lsr);
                // If data ready, also handle RX
                if (lsr & LSR_DR) {
                    handleRx(port, lsr);
                }
                break;

            case IIR_RDA:
            case IIR_TIMEOUT:
                // Received data available or FIFO timeout
                lsr = (uint8_t)_inp(base + UART_LSR);
                handleRx(port, lsr);
                break;

            case IIR_THRE:
                // Transmitter holding register empty
                handleTx(port);
                break;

            case IIR_MSR:
                // Modem status change
                handleMsr(port);
                break;
        }
    }

    // Sync queue counts into ComDEB for third-party code
    port->comDeb.qInCount  = port->rxCount;
    port->comDeb.qInHead   = port->rxHead;
    port->comDeb.qInTail   = port->rxTail;
    port->comDeb.qOutCount = port->txCount;
    port->comDeb.qOutHead  = port->txHead;
    port->comDeb.qOutTail  = port->txTail;

    checkNotify(port);
}


// -----------------------------------------------------------------------
// unhookIsr - Restore previous interrupt vector via DPMI
// -----------------------------------------------------------------------
void unhookIsr(PortStateT *port)
{
    uint8_t  intNum;
    uint8_t  picMask;
    int16_t *refCount;
    void (_far _interrupt **prevPtr)(void);

    intNum = port->irq + 8;

    if (port->irq == 3) {
        prevPtr  = &prevIsr3;
        refCount = &irq3RefCount;
    } else {
        prevPtr  = &prevIsr4;
        refCount = &irq4RefCount;
    }

    // Mask IRQ at PIC
    picMask = _inp(PIC_DATA);
    picMask |= port->irqMask;
    _outp(PIC_DATA, picMask);

    (*refCount)--;

    // Only restore vector when last user of this IRQ unhooks
    if (*refCount <= 0) {
        uint16_t oldSeg = _FP_SEG(*prevPtr);
        uint16_t oldOff = _FP_OFF(*prevPtr);

        _asm {
            mov  ax, 0205h
            mov  bl, intNum
            mov  cx, oldSeg
            mov  dx, oldOff
            int  31h
        }

        *prevPtr  = NULL;
        *refCount = 0;
    }
}


// -----------------------------------------------------------------------
// isr3 - ISR entry point for IRQ3 (COM2 and COM4)
// -----------------------------------------------------------------------
void _far _interrupt isr3(void)
{
    int16_t i;

    // Load DLL's data segment -- _interrupt saves/restores DS but doesn't
    // set it. Without this, DS belongs to whatever code was interrupted
    // and all global accesses (ports[], ring buffers) read garbage.
    _asm mov ax, seg ports
    _asm mov ds, ax

    for (i = 0; i < MAX_PORTS; i++) {
        if (ports[i].isOpen && ports[i].irq == 3) {
            isrDispatch(&ports[i]);
        }
    }

    // Send EOI to PIC
    _outp(PIC_CMD, PIC_EOI);
}


// -----------------------------------------------------------------------
// isr4 - ISR entry point for IRQ4 (COM1 and COM3)
// -----------------------------------------------------------------------
void _far _interrupt isr4(void)
{
    int16_t i;

    // Load DLL's data segment -- see comment in isr3
    _asm mov ax, seg ports
    _asm mov ds, ax

    isrHitCount++;

    for (i = 0; i < MAX_PORTS; i++) {
        if (ports[i].isOpen && ports[i].irq == 4) {
            isrDispatch(&ports[i]);
        }
    }

    // Send EOI to PIC
    _outp(PIC_CMD, PIC_EOI);
}