riscv32: move PC_START to 0x80100000 to match the mapping of qemu

src/isa/riscv32/include/isa/reg.h

@@ -4,7 +4,7 @@
 #include "common.h"
 #include "memory.h"
 
-#define PC_START IMAGE_START
+#define PC_START (0x80000000u + IMAGE_START)
 
 typedef struct {
   union {

src/isa/riscv32/init.c

@@ -3,8 +3,9 @@
 // this is not consistent with uint8_t
 // but it is ok since we do not access the array directly
 const uint32_t isa_default_img [] = {
-  0x00002023,  // sw  zero, 0(zero)
-  0x00002503,  // lw  a0,0(zero)
+  0x800002b7,  // lui t0,0x80000
+  0x0002a023,  // sw  zero,0(t0)
+  0x0002a503,  // lw  a0,0(t0)
   0x0000006b,  // nemu_trap
 };
 const long isa_default_img_size = sizeof(isa_default_img);

src/isa/riscv32/mmu.c

 #include "nemu.h"
 #include "memory/memory.h"
 
+static inline paddr_t va2pa(vaddr_t addr) {
+  return addr & ~0x80000000u;
+}
+
 uint32_t isa_vaddr_read(vaddr_t addr, int len) {
-  return paddr_read(addr, len);
+  return paddr_read(va2pa(addr), len);
 }
 
 void isa_vaddr_write(vaddr_t addr, uint32_t data, int len) {
-  paddr_write(addr, data, len);
+  paddr_write(va2pa(addr), data, len);
 }