Load memory data into a register:
LDR RD, [Rn, #offset]`LDR Rd,
Store register data to memory
Move a constant into a register
Move an address into a register
LDR Rd,=<LABEL>ADR Rd,<LABEL> (pc-relative)If label is close to the instruction, the offset is computed and stored
This is a pseudo instruction
Pre/Post Processing §
Applies to LDR, STR instructions
Also updates address register
Pre-indexed syntax: address = [Rn, #offset]
Adding in a loop example
Memory Access §
Multi-byte data ordering
E.g. num=0x1234ABCD
See Endianness
A memory access is “naturally aligned” if the address is a multiple of the data size; word-size data is stored at an address that’s a multiple of the word size, e.g. 4
E.g. if loading contents of R1 into R2, LDR R2, [R1]
Good if R1=0,4,8,12… Not good if R1=1,3,5,6, etc.
Assemblers, Linkers, Loaders §
Assemblers translate assembly into machine code (binary)
There’s a one-to-one mapping of assembly to machine instruction
Assembly directives (e.g. AREA, ALIGN, DCD) are not translated to machine instructions; they’re “config data” for the assembler
Assemblers perform 2 passes on source code (because of forward references, 2 passes is necessary):
Pass 1:
Determines instruction/data sizes
Builds a symbol table
Uses an ILC (Instruction Location Counter) to determine label adddresses
Pass 2:
Encode machine instructions
Emits a relocatable object file (.o)
Linker joins multiple compiled files together into one executable
Loader gets your program into memory so that it’s ready for execution