CS 3420: Embedded Systems

Function Calls If we want to implement POP {R4}, we first store the value in the stack pointer to R4 then increment the stack pointer by 4 because the stack grows down. We can also do POP {PC}, which is equivalent to POP {Rx} followed by BX Rx. We can’t do POP LR because LR is not a lower register. PUSH Encoding With the PUSH encoding, we encode a list of registers using a one-hot encoding to save instructions....

The C program build tool flow includes source files (.c, .h) which are compiled into object files (.o) assmebly files (.s) which are assembled into object files (.o) library files (.lib) All of these are linked into memory into an executable file (.exe, .axf). Assembly Files In assembly files, sections of (binary) instructions can be referred to using memory addresses which, in the assembly file, are called labels. In the assembled object file, the main memory locations point to each label in the assembly file....

ARM Memory Map All versions of ARM have the same memory encoding which makes compatibility easy. Each memory address is 32 bits, which means we can have 4 gigabytes of data (because each memory block holds 1 byte). Fig. 1. Memory map for Cortex M0+. The bottom 500 megabytes of memory holds the code, some of which non-volatile (256 kB), meaning if the system reboots, the memory remains. Our NXP ARM cores have a weird convention where they put one quarter of the 32 kB SRM in the code space and the rest in the SRAM code space....

ARM ISA Recap Recall that the ARM ISA has most instructions as 16 bits; since we have resource constraints and everything costs money, we want to fit as many instructions on the chip as possible. Some examples of instructions are MOV 47, #4 SUB R2, R2, R4 which is the same as SUB R2, R4 ADDS R2, R2, #3 which has the S suffix and updates the status flag Question: Which instruction(s) are not allowed to be compiled?...

Assembly Intro Assembly code is a slightly more readable version of machine code. Machine code is a list of binary instructions that are executed by the processor. In programming languages, keywords, braces, etc. make programs understandable by humans and easy to use. Compilers take that code and convert it into assembly code. Then, each line of assembly is converted in machine code. Fig. 1. One line of C code often corresponds to three or four lines of assembly code....