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 subtracts the calculated value from the accumulator.

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

Simple example

ID	Flag	Type	Size	Memory	Cmp	Type	Size	Mem/Val	Hits
1	SubSource	Mem	8-bit	0x8010
2		Mem	8-bit	0x8020	>	Value		0x04	0 (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.

ID	Flag	Type	Size	Memory	Cmp	Type	Size	Mem/Val	Hits
1	SubSource	Delta	8-bit	0x80dc
2		Mem	8-bit	0x80dc	=	Value		0x02	0 (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:

ID	Flag	Type	Size	Memory	Cmp	Type	Size	Mem/Val	Hits
1	SubSource	Mem	8-bit	0x8020
2		Mem	8-bit	0x8010	=	Value		0xffffffff	0 (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):

ID	Flag	Type	Size	Memory	Cmp	Type	Size	Mem/Val	Hits
1	SubSource	Mem	8-bit	0x8010
2		Mem	8-bit	0x8020	=	Value		1	0 (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/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:

ID	Flag	Type	Size	Memory	Cmp	Type	Size	Mem/Val	Hits
1	SubSource	Mem	32-bit	0x8020
2		Value		0xffffffe5	=	Value		0xffffffe3	0 (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.

ID	Flag	Type	Size	Memory	Cmp	Type	Size	Mem/Val	Hits
1	SubSource	Value		0xfffffffe
2		Value		0xffffffe5	=	Value		0xffffffe3	0 (0)

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

Changelog

Last 10 changes on this page:

• [2021-12-13 10:49] Jamiras: "less" -> "minus"
• [2021-11-27 08:38] Jamiras: Updated SubSource Flag (markdown)
• [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)