Loading and storing
The first thing you definitely should know when starting off with assembly, is how to load and store data using various the SNES registers. The basic opcodes for loading and storing data are LDA and STA.
As mentioned earlier, there are 3 main registers:
A (the accumulator)
X (index)
Y (index)
Although these registers can be either in 8-bit or 16-bit mode, in this tutorial we will consider them 8-bit by default.
LDA and STA
Opcode | Full name | Explanation |
LDA | Load into accumulator | Load a value into A |
STA | Store from accumulator | Store A's value into an address |
We will use RAM addresses for the sake of simplicity. Here is an example for loading and storing values.
We will look at this code line by line.
This loads the value $03 into A. The "#" means that we're loading an actual value, not an address. After this instruction, the content of the A register is now $03. LDA can load values into A, ranging from $00-$FF in 8-bit mode and $0000-$FFFF in 16-bit mode.
This stores A's value into the RAM address $7E0001. Because A's value was $03, RAM address $7E0001's value also is now $03. The contents of the A register is not cleared. This means you can chain multiple stores, like this:
A common beginner's mistake is writing STA #$7E0001 or any form of "STA #$". This instruction doesn't exist. It also doesn't make sense; there's no logic behind storing the value of A into another value.
Remember, using $ instead of #$ after an opcode means that the parameter is an address, not an immediate value.
Putting a semicolon (;) will allow everything beyond that to be ignored by the assembler, during the assembly of the code. In other words, ; is used to place comments. Example:
Loading and storing addresses
Of course, what would be the use to store things to a RAM address when you don't know how to access the address again? You can load a RAM address' contents into the A register by using LDA with a different addressing mode. Here is an example.
Again, we will look at this example line by line.
This will load the contents of the RAM address $7E0013 into A. Let's assume that the contents were $33. So now, A has the value $33. The content of $7E0013 remains unchanged, because LDA copies the number rather than extracting it from the address. Note that this time we have used $ instead of #$. This is because we wanted to access a RAM address. In the end, A has $33 and RAM $7E0013 has the value $33 also.
This instruction will store the contents of the A register into the RAM address $7E0042. Of course, A will remain unchanged. RAM address $7E0042 is now $33. In short: the full example will copy the contents of $7E0013 over to $7E0042.
LDY, STY, LDX, STX
Now that we have learned the basics of loading and store values into addresses, let's introduce the same loading opcodes, but for the index registers:
Opcode | Full name | Explanation |
LDY | Load into Y | Load a value into Y |
STY | Store from Y | Store Y's value into an address |
LDX | Load into X | Load a value into X |
STX | Store from X | Store X's value into an address |
The above opcodes behave exactly like LDA and STA. The only difference is that these make use of the X and the Y registers instead of the accumulator. For example:
Would store the number $03 into RAM address $7E0001, by utilizing the Y register. To use the X register, use LDX and STX. As for why the address is $0001 instead of $7E0001, please refer to this chapter: Shorter addresses
STZ
There is another opcode which stores the number $00 into addresses directly.
Opcode | Full name | Explanation |
STZ | Store zero to memory | Sets the value of an address to 0 |
This opcode stores the number $00 into an address. It doesn't even need the A, X or Y registers to load $00 first.
If you want to make a code that directly stores $00 in a RAM address, you could make it use 1 line:
STZ will store zero to a specified RAM address. After this opcode, RAM address $7E0001 will now contain the number $00. Using STZ when A is in 16-bit mode will store $0000 to both RAM addresses $7E0001 and $7E0002.
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