1 EECS 373 Design of Microprocessor-Based Systems Prabal Dutta University of Michigan Lecture 4: Memory and Memory-Mapped I/O September 16, 2010

2 Announcements Prabal’s office hours this week –Thursday, 9/16 –1:00 PM to 2:30 PM –4773 CSE Homework 1 to be posted

3 Outline Minute quiz Announcements Review Embedded C/Assembly Memory Memory-mapped I/O

What happens after a power-on-reset (POR)? On the ARM Cortex-M3 SP and PC are loaded from the code (.text) segment Initial stack pointer –LOC: 0x –POR: SP  mem(0x ) Interrupt vector table –Initial base: 0x –Vector table is relocatable –Entries: 32-bit values –Each entry is an address –Entry #1: reset vector LOC: 0x POR: PC  mem(0x ) Execution begins 4.equSTACK_TOP, 0x text.syntaxunified.thumb.global_start.typestart, %function _start:.wordSTACK_TOP, start start: movs r0, #10...

5 How does an assembly language program get turned into a executable program image? Assembly files (.s) Object files (.o) as (assembler) ld (linker) Memory layout Memory layout Linker script (.ld) Executable image file Binary program file (.bin) Disassembled code (.lst) objcopy objdump

6 How does a mixed C/Assembly program get turned into a executable program image? Assembly files (.s) Object files (.o) as (assembler) gcc (compile + link) Memory layout Memory layout Linker script (.ld) Executable image file Binary program file (.bin) Disassembled Code (.lst) objcopy objdump ld (linker) Library object files (.o) C files (.c)

7 Outline Minute quiz Announcements Review Embedded C/Assembly Memory Memory-mapped I/O

8 #define SYSREG_SOFT_RST_CR 0xE uint32_t *reg = (uint32_t *)(SYSREG_SOFT_RST_CR); main () { *reg |= 0x ; // Reset GPIO hardware *reg &= ~(0x ); } Accessing memory locations from C Memory has an address and value Can equate a pointer to desired address Can set/get de-referenced value to change memory

9 Some useful C keywords const –Makes variable value or pointer parameter unmodifiable –const foo = 32; register –Tells compiler to locate variables in a CPU register if possible –register int x; static –Preserve variable value after its scope ends –Does not go on the stack –static int x; volatile –Opposite of const –Can be changed in the background –volatile int I;

10 Homework Write a library in assembly. Make it compatible with the EABI Link it with C

11 Outline Minute quiz Announcements Review Embedded C/Assembly Memory Memory-mapped I/O

System Memory Map

13 Outline Minute quiz Announcements Review Embedded C/Assembly Memory Memory-mapped I/O

14 Questions? Comments? Discussion?

15 int main() { int i; int n; unsigned int input = 40, output = 0; for (i = 0; i < 10; ++i) { n = factorial(i); printf("factorial(%d) = %d\n", i, n); } __asm("nop\n"); __asm("mov r0, %0\n" "mov r3, #5\n" "udiv r0, r0, r3\n" "mov %1, r0\n" :"=r" (output) : "r" (input) : "cc", "r3" ); __asm("nop\n"); printf("%d\n", output); } Cheap trick: use asm() or __asm() macros to sprinkle simple assembly in standard C code! $ arm-none-eabi-gcc \ -mcpu=cortex-m3 \ -mthumb main.c \ -T generic-hosted.ld \ -o factorial $ qemu-arm -cpu cortex-m3 \./factorial factorial(0) = 1 factorial(1) = 1 factorial(2) = 2 factorial(3) = 6 factorial(4) = 24 factorial(5) = 120 factorial(6) = 720 factorial(7) = 5040 factorial(8) = factorial(9) = Answer: 40/5 