- Opcode: 0x14
- Short name: IFUB
- Long name: If (Unsigned Byte)
Memory layout
| 0x14 | B1 / B2 | A | V | C | E | |——|———–|—–|—–|—–|—–|
Arguments
- const Bit[4] B1: First memory bank to access.
- const Bit[4] B2: Second memory bank to access.
- const UByte A: Address, from the first bank, of the value to retrieve.
- const UByte V: Unsigned value to compare the retrieved value to, or address from the second bank of the value to retrieve.
- const UByte C: Type of comparison to perform.
- const UByte E: Amount to jump if the comparison does not hold.
Description
Performs a comparison between a retrieved values from memory at specific banks and addresses. If B2 is zero, then the value retrieved from bank B1, address A is compared with the value specified by V. If B2 is not zero, then the value retrieved from bank B1, address A is compared with the value retrieved from bank B2, address V.
The type of comparison used is shown in the table below. If the comparison fails, the script pointer jumps forward by the amount specified in the final argument; the starting offset for this jump is just before the jump value argument itself. This, combined with an appropriate jump forward, is used to implement if/else functionality in scripts, as demonstrated below.
If the content of the ‘if’ block following the IFUB line is longer than 0xFF, the jump argument will also need to be longer than 0xFF, and hence the IFUBL variant is used instead. If the value to compare is larger than 0xFF, the IFUW/IFUWL variants are used. If the value being compared is negative, the IFSW/IFSWL variants are used.
Comparison Types
| ID | Comparison Performed |
|---|---|
| 0 | A == B |
| 1 | A != B |
| 2 | A > B |
| 3 | A < B |
| 4 | A >= B |
| 5 | A <= B |
| 6 | A & B |
| 7 | A ^ B |
| 8 | A | B |
| 9 | A & (1<<B) |
| A | !((A & (1<<B))) |
If/Else Example
In this example, if the value found is greater than 0x30, the script halts for a shorter period of time; otherwise, the script halts for a long period of time. Note how the jump forward placed before the longer WAIT (start of the required ‘else’ block) ensures that on completion of the ‘if’ block, execution of the ‘else’ block is avoided. It jumps forward to just before the 0x200 WAIT, which it then executes, and returns.
The ‘else’ block only executes if it is jumped into by the final argument of the IFUB argument list (thus avoiding the execution of the ‘if’ block). Once the ‘else’ block is executed, script execution just continues and the 0x200 WAIT and RET is executed. Since this is also executed by the ‘if’ block having jumped forward to skip the else block, this demonstrates that the two code paths converge. In C++ (below) this is the equivalent of the if/else block being completed.
IFUB (10,20,30,02,06)
WAIT (00,01)
JMPF (04)
WAIT (00,04)
WAIT (00,02)
RET ()