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When selecting a texture it is similar to C but is far more simplified. Normally, in C you would do the following:
#define Texture 0
In asm this translates to the following:
mov R7, 0 out GPU_SelectedTexture, R7
What is happening in the above snip of code is that we are putting in our number ID for our texture into register 7 and then feeding R7 into the GPU_SelectedTexture.
Implemented from scratch, in assembly: