wow, preimage oracle was so easy to write

contracts/MIPS.sol

@@ -43,11 +43,30 @@ contract MIPS {
   }
 
   function ReadMemory(bytes32 stateHash, uint32 addr) public view returns (uint32) {
+    require(addr & 3 == 0, "read memory must be 32-bit aligned");
+
+    // zero register is always 0
     if (addr == REG_OFFSET) {
-      // zero register is always 0
       return 0;
     }
-    require(addr & 3 == 0, "read memory must be 32-bit aligned");
+
+    // MMIO preimage oracle
+    if (addr >= 0x31000000 && addr < 0x32000000) {
+      bytes32 pihash = ReadBytes32(stateHash, 0x30001000);
+      if (addr == 0x31000000) {
+        return uint32(preimage[pihash].length);
+      }
+      uint offset = addr-0x31000004;
+      uint8 a0 = uint8(preimage[pihash][offset]);
+      uint8 a1 = uint8(preimage[pihash][offset+1]);
+      uint8 a2 = uint8(preimage[pihash][offset+2]);
+      uint8 a3 = uint8(preimage[pihash][offset+3]);
+      return (uint32(a0) << 24) |
+             (uint32(a1) << 16) |
+             (uint32(a2) << 8) |
+             (uint32(a3) << 0);
+    }
+
     uint64 ret = state[stateHash][addr];
     require((ret >> 32) == 1, "memory was not initialized");
     return uint32(ret);

risc/run.py

@@ -30,7 +30,6 @@ heap_start = 0x20000000 # 0x20000000-0x30000000
 # output oracle             @ 0x30000800
 # preimage oracle (write)   @ 0x30001000
 # preimage oracle (read)    @ 0x31000000-0x32000000 (16 MB)
-# preimage oracle (trigger) @ 0x32000000 (returns size)
 
 brk_start = 0x40000000  # 0x40000000-0x80000000