To load the trainer ID, you would do the following:
.align 2
.thumb
ldr r0, =(0x0300500c)
ldr r0, [r0]
ldrh r0, [r0, #0xa]
The difference is "ldrh" vs. "ldr." The former loads a halfword (2 bytes) and can do so in multiplies of two bytes, whereas the latter loads a full word (4 bytes) and...