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SubSource Flag

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 Add Source, but the Sub Source flag makes the value in the memory address have a negative value.

Note 1: Sub Source is NOT a subtraction flag. It just makes the value be negative.

Note 2: The final line (without Add Source or Sub Source) is still added for the final comparison.

Using that AddSource usecase example, if we replaced it with the Sub Source and with the same values (value(0x8010) = 1 and value(0x8020) = 2), the comparison would be -1 + 2 > 0, or 1 > 0.

Using SubSource to Count Specific Incrementslink

SubSource can also be used to track specific increases in addresses. The conditions below states that the current value in address 0x0080dc less 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 0x0080dc increases by exactly two.

subsource

Checking for a negative resultlink

If the result of your Sub Source operation is a negative number, it is possible to check for it. This is how negative numbers are represented:

  • -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, let's say you want to check if value(0x8010) = 1 minus value(0x8020) = 2 equals -1. This is how you would do it:

Sub Source negative numbers

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

Advanced: it is also possible to use straight negative numbers in the left section of your Sub/Add Source 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:

Sub Source negative numbers 2

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

Take care, though, if you use a straight negative number in a Sub Source flag, it will result in a double negative, turning the number back to positive. Take a look:

Sub Source negative numbers 3

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

Changeloglink

Last 10 changes on this page:

  • [2020-01-15 08:23] meleu: better example of a negative number
  • [2019-12-31 12:14] meleu: cosmetic
  • [2019-02-17 23:42] meleu: better example for "Using SubSource to Count Specific Increments"
  • [2019-02-17 21:05] meleu: cosmetic
  • [2019-02-14 10:10] meleu: fix broken link
  • [2019-02-12 13:25] meleu: add heading to "Using SubSource to Count Increments"
  • [2019-02-12 13:18] meleu: fix broken link
  • [2019-02-12 09:37] meleu: Updated SubSource (markdown)