Ldr r2, load the value (0x03) at memory address found in R0 to register R2 Ldr r1, adr_var2 load the memory address of var2 via label adr_var2 into R1 Ldr r0, adr_var1 load the memory address of var1 via label adr_var1 into R0 text /* start of the text (code) section */ data section is dynamically created and its addresses cannot be easily predicted */ This is how it would look like in a functional assembly program. STR operation: stores the value found in R2 to the memory address found in R1. LDR operation: loads the value at the address found in R0 to the destination register R2. STR R2, - destination address is the value found in R1. LDR R2, - origin address is the value found in R0. Generally, LDR is used to load something from memory into a register, and STR is used to store something from a register to a memory address. Offset form: Scaled register as the offset.Offset form : Immediate value as the offset.
ARM EMULATOR CODE EXAMPLE CODE
The best way to follow this part of the tutorial is to run the code examples in a debugger (GDB) on your lab environment. For each example we will use the same piece of assembly code with a different LDR/STR offset form, to keep it simple. To explain the fundamentals of Load and Store operations on ARM, we start with a basic example and continue with three basic offset forms with three different address modes for each offset form. This means that incrementing a 32-bit value at a particular memory address on ARM would require three types of instructions (load, increment, and store) to first load the value at a particular address into a register, increment it within the register, and store it back to the memory from the register. While on x86 most instructions are allowed to directly operate on data in memory, on ARM data must be moved from memory into registers before being operated on. ARM uses a load-store model for memory access which means that only load/store (LDR and STR) instructions can access memory.
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