Added UART detection and reporting.

rs232/README.md

@@ -10,6 +10,7 @@ stripped to DJGPP-only codepaths and restyled.
 
 - ISR-driven receive and transmit with 2048-byte ring buffers
 - Auto-detected IRQ from BIOS data area
+- UART type detection (8250, 16450, 16550, 16550A)
 - 16550 FIFO detection and configurable trigger threshold
 - XON/XOFF, RTS/CTS, and DTR/DSR flow control
 - DPMI memory locking for ISR safety
@@ -29,6 +30,16 @@ All functions take a COM port index (`int com`) as their first argument:
 | `RS232_COM3` | 2     | COM3        |
 | `RS232_COM4` | 3     | COM4        |
 
+### UART Types
+
+| Constant            | Value | Description                          |
+|---------------------|-------|--------------------------------------|
+| `RS232_UART_UNKNOWN`| 0     | Unknown or undetected                |
+| `RS232_UART_8250`   | 1     | 8250 — no FIFO, no scratch register  |
+| `RS232_UART_16450`  | 2     | 16450 — scratch register, no FIFO    |
+| `RS232_UART_16550`  | 3     | 16550 — broken FIFO (unusable)       |
+| `RS232_UART_16550A` | 4     | 16550A — working 16-byte FIFO        |
+
 ### Handshaking Modes
 
 | Constant                 | Value | Description               |
@@ -138,8 +149,18 @@ int     rs232GetRts(int com);        // RTS line state (0 or 1)
 int     rs232GetRxBuffered(int com); // Bytes in receive buffer
 int     rs232GetStop(int com);       // Stop bits setting
 int     rs232GetTxBuffered(int com); // Bytes in transmit buffer
+int     rs232GetUartType(int com);  // UART type (RS232_UART_* constant)
 ```
 
+`rs232GetUartType` probes the UART hardware to identify the chip:
+
+1. **Scratch register test** — writes two values to register 7 and
+   reads them back. The 8250 lacks this register, so readback fails.
+2. **FIFO test** — enables the FIFO via the FCR, then reads IIR bits
+   7:6. `0b11` = 16550A (working FIFO), `0b10` = 16550 (broken FIFO),
+   `0b00` = 16450 (no FIFO). The original FCR value is restored after
+   probing.
+
 #### Setters
 
 ```c
@@ -175,6 +196,10 @@ int main(void) {
         return 1;
     }
 
+    // Identify UART chip
+    int uartType = rs232GetUartType(RS232_COM1);
+    // uartType == RS232_UART_16550A on most systems
+
     // Blocking send
     rs232Write(RS232_COM1, "Hello\r\n", 7);
 

rs232/rs232.c

@@ -107,6 +107,7 @@
 #define UART_MCR                4
 #define UART_LSR                5
 #define UART_MSR                6
+#define UART_SCR                7 // scratch register (not present on 8250)
 
 // UART read macros
 #define UART_READ_DATA(C)       inportb((C)->base + UART_RX)
@@ -955,6 +956,49 @@ int rs232GetTxBuffered(int com) {
 }
 
 
+int rs232GetUartType(int com) {
+    Rs232StateT *port = &sComPorts[com];
+    uint8_t      scratch;
+    uint8_t      iir;
+
+    if (com < COM_MIN || com > COM_MAX) {
+        return RS232_ERR_INVALID_PORT;
+    }
+    if (!port->isOpen) {
+        return RS232_ERR_NOT_OPEN;
+    }
+
+    // Test scratch register — 8250 lacks it
+    outportb(port->base + UART_SCR, 0xA5);
+    scratch = inportb(port->base + UART_SCR);
+    if (scratch != 0xA5) {
+        return RS232_UART_8250;
+    }
+    outportb(port->base + UART_SCR, 0x5A);
+    scratch = inportb(port->base + UART_SCR);
+    if (scratch != 0x5A) {
+        return RS232_UART_8250;
+    }
+
+    // Has scratch register — at least 16450. Try enabling FIFO.
+    outportb(port->base + UART_FCR, FCR_ENABLE);
+    iir = inportb(port->base + UART_IIR);
+
+    // Restore original FCR
+    outportb(port->base + UART_FCR, port->fcr);
+
+    // Check IIR bits 7:6 for FIFO status
+    switch (iir & 0xC0) {
+        case 0xC0:
+            return RS232_UART_16550A;
+        case 0x80:
+            return RS232_UART_16550;
+        default:
+            return RS232_UART_16450;
+    }
+}
+
+
 int rs232Open(int com, int32_t bps, int dataBits, char parity, int stopBits, int handshake) {
     Rs232StateT *port = &sComPorts[com];
     int          rc   = RS232_SUCCESS;

rs232/rs232.h

@@ -22,6 +22,13 @@
 #define RS232_HANDSHAKE_RTSCTS          2
 #define RS232_HANDSHAKE_DTRDSR          3
 
+// UART Types
+#define RS232_UART_UNKNOWN              0
+#define RS232_UART_8250                 1
+#define RS232_UART_16450                2
+#define RS232_UART_16550                3
+#define RS232_UART_16550A               4
+
 // Error Codes
 #define RS232_SUCCESS                   0
 #define RS232_ERR_UNKNOWN              -1
@@ -67,6 +74,7 @@ int     rs232GetRts(int com);
 int     rs232GetRxBuffered(int com);
 int     rs232GetStop(int com);
 int     rs232GetTxBuffered(int com);
+int     rs232GetUartType(int com);
 
 // Read/write
 int rs232Read(int com, char *data, int len);