ClearAll[memory, MemoryGet, MemorySet, MemorySubtract]
memory = {15, 17, -1, 17, -1, -1, 16, 1, -1, 16, 3, -1, 15, 15, 0,
   0, -1, 72, 101, 108, 108, 111, 44, 32, 119, 111, 114, 108, 100, 33,
    10, 0};
MemoryGet[addr_] := memory[[addr + 1]]
MemorySet[addr_, value_] := memory[[addr + 1]] = value
MemorySubtract[addr1_, addr2_] := MemorySet[addr1, MemoryGet[addr1] - MemoryGet[addr2]]
p = 0;
While[p >= 0,
 a = MemoryGet[p];
 b = MemoryGet[p + 1];
 c = MemoryGet[p + 2];
 If[b == -1,
  Print[FromCharacterCode[MemoryGet[a]]]
  ,
  MemorySubtract[b, a];
  If[MemoryGet[b] < 1,
   p = MemoryGet[p + 2];
   Continue[]
   ]
  ];
 p += 3;
 ]