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--- Part Two ---
For some reason, the sea port's computer system still can't communicate with your ferry's docking
program. It must be using [1m[37mversion 2[0m of the decoder chip!
A version 2 decoder chip doesn't modify the values being written at all. Instead, it acts as a
memory address decoder. Immediately before a value is written to memory, each bit in the bitmask
modifies the corresponding bit of the destination [1m[37mmemory address[0m in the following way:
- If the bitmask bit is 0, the corresponding memory address bit is [1m[37munchanged[0m.
- If the bitmask bit is 1, the corresponding memory address bit is [1m[37moverwritten with 1[0m.
- If the bitmask bit is X, the corresponding memory address bit is [1m[37mfloating[0m.
A [1m[37mfloating[0m bit is not connected to anything and instead fluctuates unpredictably. In
practice, this means the floating bits will take on [1m[37mall possible values[0m, potentially
causing many memory addresses to be written all at once!
For example, consider the following program:
mask = 000000000000000000000000000000X1001X
mem[42] = 100
mask = 00000000000000000000000000000000X0XX
mem[26] = 1
When this program goes to write to memory address 42, it first applies the bitmask:
address: 000000000000000000000000000000101010 (decimal 42)
mask: 000000000000000000000000000000X1001X
result: 000000000000000000000000000000[1m[37mX1[0m10[1m[37m1X[0m
After applying the mask, four bits are overwritten, three of which are different, and two of which
are [1m[37mfloating[0m. Floating bits take on every possible combination of values; with two
floating bits, four actual memory addresses are written:
000000000000000000000000000000[1m[37m0[0m1101[1m[37m0[0m (decimal 26)
000000000000000000000000000000[1m[37m0[0m1101[1m[37m1[0m (decimal 27)
000000000000000000000000000000[1m[37m1[0m1101[1m[37m0[0m (decimal 58)
000000000000000000000000000000[1m[37m1[0m1101[1m[37m1[0m (decimal 59)
Next, the program is about to write to memory address 26 with a different bitmask:
address: 000000000000000000000000000000011010 (decimal 26)
mask: 00000000000000000000000000000000X0XX
result: 00000000000000000000000000000001[1m[37mX[0m0[1m[37mXX[0m
This results in an address with three floating bits, causing writes to [1m[37meight[0m memory
addresses:
00000000000000000000000000000001[1m[37m0[0m0[1m[37m00[0m (decimal 16)
00000000000000000000000000000001[1m[37m0[0m0[1m[37m01[0m (decimal 17)
00000000000000000000000000000001[1m[37m0[0m0[1m[37m10[0m (decimal 18)
00000000000000000000000000000001[1m[37m0[0m0[1m[37m11[0m (decimal 19)
00000000000000000000000000000001[1m[37m1[0m0[1m[37m00[0m (decimal 24)
00000000000000000000000000000001[1m[37m1[0m0[1m[37m01[0m (decimal 25)
00000000000000000000000000000001[1m[37m1[0m0[1m[37m10[0m (decimal 26)
00000000000000000000000000000001[1m[37m1[0m0[1m[37m11[0m (decimal 27)
The entire 36-bit address space still begins initialized to the value 0 at every address, and you
still need the sum of all values left in memory at the end of the program. In this example, the sum
is [1m[37m208[0m.
Execute the initialization program using an emulator for a version 2 decoder chip. [1m[37mWhat is
the sum of all values left in memory after it completes?[0m
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