diff --git a/Interpreter.cpp b/Interpreter.cpp
index bea55aa..2992a86 100644
--- a/Interpreter.cpp
+++ b/Interpreter.cpp
@@ -71,7 +71,7 @@ void Interpreter::tick() {
 
   // fetch instruction
   sreg_t iInstAddr = mvSpecialReg[SR_PC];
-  unsigned inst = (mvMemory[iInstAddr] << 8) | mvMemory[iInstAddr + 1];
+  unsigned inst = (mvMemory.at(iInstAddr) << 8) | mvMemory.at(iInstAddr + 1);
 
   // increment program counter
   mvSpecialReg[SR_PC] += 2;
@@ -163,18 +163,18 @@ void Interpreter::tick() {
       mvSpecialReg[SR_I] = scLowRestFontAddr + 5 * mvReg[iRegDst];
       break;
     case 0xF033: // FX33 - convert to bcd
-      mvMemory[mvSpecialReg[SR_I]]     = (mvReg[iRegDst] / 100) % 10;
-      mvMemory[mvSpecialReg[SR_I] + 1] = (mvReg[iRegDst] / 10) % 10; 
-      mvMemory[mvSpecialReg[SR_I] + 2] = mvReg[iRegDst] % 10; 
+      mvMemory.at(mvSpecialReg[SR_I])     = (mvReg[iRegDst] / 100) % 10;
+      mvMemory.at(mvSpecialReg[SR_I] + 1) = (mvReg[iRegDst] / 10) % 10;
+      mvMemory.at(mvSpecialReg[SR_I] + 2) = mvReg[iRegDst] % 10;
       break;
     case 0xF055: // FX55 - dump registers
       for(int i = 0; i <= iRegDst - R_V0; i++) {
-        mvMemory[mvSpecialReg[SR_I]++] = mvReg[R_V0 + i];
+        mvMemory.at(mvSpecialReg[SR_I]++) = mvReg[R_V0 + i];
       }
       break;
     case 0xF065: // FX65 - restore registers
       for(int i = 0; i <= iRegDst - R_V0; i++) {
-        mvReg[R_V0 + i] = mvMemory[mvSpecialReg[SR_I]++];
+        mvReg[R_V0 + i] = mvMemory.at(mvSpecialReg[SR_I]++);
       }
       break;
     default:
@@ -272,14 +272,14 @@ void Interpreter::executeDraw(uint8_t x, uint8_t y, uint8_t n) {
     uint16_t spriteRowBits;
     if(mIsHighResMode) {
       // draws an 8xN sprite
-      spriteRowBits = mvMemory[iMemAddr++] << 8;
+      spriteRowBits = mvMemory.at(iMemAddr++) << 8;
       if(n == 0) {
         // draws an 16xN sprite, so fetch another byte from sprite data
-        spriteRowBits |= mvMemory[iMemAddr++];
+        spriteRowBits |= mvMemory.at(iMemAddr++);
       }
     } else {
       // in low-res mode, each sprite pixel draws two on-screen pixels
-      spriteRowBits = gcvLowResToHighResRowLookupTable[mvMemory[iMemAddr++]];
+      spriteRowBits = gcvLowResToHighResRowLookupTable[mvMemory.at(iMemAddr++)];
     }
 
     // convert to bitset and shift into absolute horizontal position on the screen