Clean up/optimize Debug ROM.

It's 3 instructions shorter now.

debug_rom/debug_rom.S

@@ -12,11 +12,10 @@
         // their GO or RESUME bit is set.
 
 entry:
-       jal zero, _entry
+        jal zero, _entry
-resume:
-       jal zero, _resume
 exception:
-       jal zero, _exception
+        sw      zero, DEBUG_ROM_EXCEPTION(zero) // Let debug module know you got an exception.
+        // fall through to _entry
 
 _entry:
         // This fence is required because the execution may have written something
@@ -40,10 +39,6 @@ entry_loop:
         bnez s0, resume
         jal  zero, entry_loop
 
-_exception:
-        sw      zero, DEBUG_ROM_EXCEPTION(zero) // Let debug module know you got an exception.
-        ebreak
-
 going:
         csrr s0, CSR_DSCRATCH            // Restore s0 here
         sw zero, DEBUG_ROM_GOING(zero)   // When debug module sees this write, the GO flag is reset.
@@ -53,7 +48,7 @@ going:
                                          // so we use fence and fence.i for safety. (rocket-chip doesn't have this
                                          // because jalr is special there)
 
-_resume:
+resume:
         csrr s0, CSR_MHARTID
         sw   s0, DEBUG_ROM_RESUMING(zero) // When Debug Module sees this write, the RESUME flag is reset.
         csrr s0, CSR_DSCRATCH   // Restore s0

debug_rom/debug_rom.h

@@ -1,12 +1,11 @@
 static const unsigned char debug_rom_raw[] = {
-  0x6f, 0x00, 0xc0, 0x00, 0x6f, 0x00, 0x40, 0x05, 0x6f, 0x00, 0x40, 0x03,
+  0x6f, 0x00, 0x80, 0x00, 0x23, 0x26, 0x00, 0x10, 0x0f, 0x00, 0xf0, 0x0f,
-  0x0f, 0x00, 0xf0, 0x0f, 0x73, 0x10, 0x24, 0x7b, 0x73, 0x24, 0x40, 0xf1,
+  0x73, 0x10, 0x24, 0x7b, 0x73, 0x24, 0x40, 0xf1, 0x23, 0x20, 0x80, 0x10,
-  0x23, 0x20, 0x80, 0x10, 0x03, 0x44, 0x04, 0x40, 0x13, 0x74, 0x14, 0x00,
+  0x03, 0x44, 0x04, 0x40, 0x13, 0x74, 0x14, 0x00, 0x63, 0x1c, 0x04, 0x00,
-  0x63, 0x10, 0x04, 0x02, 0x73, 0x24, 0x40, 0xf1, 0x03, 0x44, 0x04, 0x40,
+  0x73, 0x24, 0x40, 0xf1, 0x03, 0x44, 0x04, 0x40, 0x13, 0x74, 0x24, 0x00,
-  0x13, 0x74, 0x24, 0x00, 0xe3, 0x18, 0x04, 0xfc, 0x6f, 0xf0, 0xdf, 0xfd,
+  0x63, 0x1e, 0x04, 0x00, 0x6f, 0xf0, 0xdf, 0xfd, 0x73, 0x24, 0x20, 0x7b,
-  0x23, 0x26, 0x00, 0x10, 0x73, 0x00, 0x10, 0x00, 0x73, 0x24, 0x20, 0x7b,
   0x23, 0x22, 0x00, 0x10, 0x0f, 0x00, 0xf0, 0x0f, 0x0f, 0x10, 0x00, 0x00,
   0x67, 0x00, 0x00, 0x30, 0x73, 0x24, 0x40, 0xf1, 0x23, 0x24, 0x80, 0x10,
   0x73, 0x24, 0x20, 0x7b, 0x73, 0x00, 0x20, 0x7b
 };
-static const unsigned int debug_rom_raw_len = 104;
+static const unsigned int debug_rom_raw_len = 92;

riscv/debug_rom_defines.h

@@ -18,6 +18,6 @@
 // These needs to match the link.ld         
 #define DEBUG_ROM_WHERETO 0x300
 #define DEBUG_ROM_ENTRY   0x800
-#define DEBUG_ROM_TVEC    0x808
+#define DEBUG_ROM_TVEC    0x804
 
 #endif