first semihosting integration

libwrap/semihosting/trap.c

@@ -0,0 +1,60 @@
+#include <stdint.h>
+#include <stdlib.h>
+#include <math.h>
+
+#include "encoding.h"
+
+#define EBREAK_OPCODE   0x00100073
+#define EBREAK_MCAUSE   0x00000003
+
+#define SLLI_X0_X0_0X1F_OPCODE  0x01f01013
+#define SRAI_X0_X0_0X07_OPCODE  0x40705013
+
+int sh_missing_host = 0;
+
+void trap()
+{
+    uint32_t mepc   = read_csr(0x341);       // Address of trap
+    uint32_t mtval  = read_csr(0x343);      // Instruction value of trap
+    uint32_t mcause = read_csr(0x342);     // Reason for the trap
+
+    if (mcause == EBREAK_MCAUSE && mtval == EBREAK_OPCODE){
+        // This trap was caused by an EBREAK...
+
+        int aligned = ((mepc-4) & 0x0f) == 0;
+        if (aligned 
+            && *(uint32_t *)mepc     == EBREAK_OPCODE 
+            && *(uint32_t *)(mepc-4) == SLLI_X0_X0_0X1F_OPCODE
+            && *(uint32_t *)(mepc+4) == SRAI_X0_X0_0X07_OPCODE)
+        {
+            // The EBREAK was part of the semihosting call. (See semihosting.c)
+            //
+            // If a debugger were connected, this would have resulted in a CPU halt,
+            // and the debugger would have serviced the the semihosting call.
+            // 
+            // However, the semihosting function was called without a debugger being 
+            // attached. The best course of action is to simply return from the trap 
+            // and let the semihosting function continue after the call to EBREAK to 
+            // prevent the CPU from hanging in the trap handler. 
+            write_csr(mepc, mepc+4);
+
+            // Set a global variable to tell the semihosting code the the semihosting 
+            // call
+            // didn't execute on the host.
+            sh_missing_host = 1;
+
+            return;
+        }
+
+        // EBREAK was not part of a semihosting call. This should not have happened. 
+        // Hang forever.
+        while(1)
+            ;
+    }
+
+    // Trap was issued for another reason than an EBREAK.
+    // Replace the code below with whatever trap handler you'd normally use. (e.g. interrupt 
+    // processing.)
+    while(1)
+        ;
+}