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Note: most likely this technique is not needed for simple games. So, it's not mandatory for jr-devs.

The SubSource can be chosen in the Flag column of the Achievement Editor

Works similarly to AddSource, but the SubSource flag subtracts the calculated value from the accumulator.

Because it subtracts from the accumulator, SubSource A, B is actually B - A.

Simple example

2Mem8-bit0x8020>Value0x040 (0)

In this example the value in 0x8010 will be subtracted from the value in 0x8020 and the comparison will check is if this sum is greater than four.

If the value in 0x8010 is 1 and the value in 0x8020 is 2, the comparison will be -1 + 2 > 4, or 1 > 4.

Using SubSource to Count Specific Increments

SubSource can also be used to track specific increases in addresses. The conditions below states that the current value in address 0x80dc minus the value in the previous frame in the same address must be equal 2. Effectively, this means that the condition is true whenever the value in address 0x80dc increases by exactly two.

2Mem8-bit0x80dc=Value0x020 (0)

Checking for a negative result

The accumulator and all logic performed is limited to an unsigned 32-bit number. If the total drops below 0, it underflows, resulting in a very large positive number. Here's the representation for the first few negative numbers:

  • -1 is 0xFFFFFFFF
  • -2 is 0xFFFFFFFE
  • -3 is 0xFFFFFFFD
  • -4 is 0xFFFFFFFC
  • -5 is 0xFFFFFFFB
  • -6 is 0xFFFFFFFA
  • -7 is 0xFFFFFFF9
  • and so on...

So, if you wanted to see if a value was 1 less than another value, you could write:

2Mem8-bit0x8010=Value0xffffffff0 (0)

This would be true if the value at $8010 (1) was 1 less than the value at $8020 (2): 1 - 2 = -1 = 0xFFFFFFFF.

However, you should swap the conditions to make it easier to read (B - A = -1) is the same as (A - B = 1):

2Mem8-bit0x8020=Value10 (0)

This evaluates -1 + 2 = 1, which is true.

Subtracting negative constants

It is also possible to use straight negative numbers in the left section of your Sub/AddSource operations. To do so, remember to change the Type from Mem to Value, and use the values as explained above, such as 0xFFFFFFE5 for -27, and 0xFFFFFFE3 for -29, for example:

2Value0xffffffe5=Value0xffffffe30 (0)

If $8020 contains 2, this would be true. -2 + (-27) = -29, which is -2 - 27 = -29, which is true.

If you use a negative number in a SubSource flag, it will result in a double negative, turning the number back to positive.

2Value0xffffffe5=Value0xffffffe30 (0)

This means -(-2) + (-27) = -29, which is 2 - 27 = -29, which is -25 = -29, which is false.

Using Modifiers

Like AddSource, you can click the "Cmp" column to add a modifier to perform and operation that modifies the value before subtracting it from the accumulator. See Add Source - Using Modifiers for more details.

Released under the GPL-3 License. There are no copyright-protected ROMs available for download on RetroAchievements.