Bare metal, Assembly language • Trouble with I2C and LCD setup with a custom OS

Code:

#include <stddef.h>#include <stdint.h>// The GPIO registers base address.#define I2C_BASE 0x3F804000  // Adjust based on Pi version  // all different regesters for reading and writing different things.#define I2C_C    *(volatile uint32_t *)(I2C_BASE + 0x00)  // C= Control Regester#define I2C_S    *(volatile uint32_t *)(I2C_BASE + 0x04)  // S= Status Regester#define I2C_DLEN *(volatile uint32_t *)(I2C_BASE + 0x08)  // DLEN= Data Length#define I2C_A    *(volatile uint32_t *)(I2C_BASE + 0x0C)  // A= Slave regester, the 1st 7 bits of which contain the address of the I2C component (unique to each device, it depends.) for LCD displays, usualy 0x27, but can be 0x3F rarely#define I2C_FIFO *(volatile uint32_t *)(I2C_BASE + 0x10)  // FIFO= first in first out, very small amount of storage/* enum {  // Probobly not going to use this as I can't find a ttc to usb, but good just in case  GPPUD = (GPIO_BASE + 0x94),  GPPUDCLK0 = (GPIO_BASE + 0x98),  // The base address for UART.  UART0_BASE = 0x3F201000, // for raspi2 & 3, 0x20201000 for raspi1  UART0_DR     = (UART0_BASE + 0x00),  UART0_RSRECR = (UART0_BASE + 0x04),  UART0_FR     = (UART0_BASE + 0x18),  UART0_ILPR   = (UART0_BASE + 0x20),  UART0_IBRD   = (UART0_BASE + 0x24),  UART0_FBRD   = (UART0_BASE + 0x28),  UART0_LCRH   = (UART0_BASE + 0x2C),  UART0_CR     = (UART0_BASE + 0x30),  UART0_IFLS   = (UART0_BASE + 0x34),  UART0_IMSC   = (UART0_BASE + 0x38),  UART0_RIS    = (UART0_BASE + 0x3C),  UART0_MIS    = (UART0_BASE + 0x40),  UART0_ICR    = (UART0_BASE + 0x44),  UART0_DMACR  = (UART0_BASE + 0x48),  UART0_ITCR   = (UART0_BASE + 0x80),  UART0_ITIP   = (UART0_BASE + 0x84),  UART0_ITOP   = (UART0_BASE + 0x88),  UART0_TDR    = (UART0_BASE + 0x8C),}; */#define GPIO_BASE      0x3F200000  // for raspi2 & 3, 0x20200000 for raspi1  // turn gpio base into an address (pointer) to be writen to with a new versoin of the variable that is now lowercase#define GPFSEL0        0x00#define GPFSEL1        0x04#define GPFSEL2        0x08#define GPFSEL3        0x0c#define GPFSEL4        0x10#define GPFSEL5        0x14#define GPFSET0        *(volatile uint32_t *)(GPIO_BASE + 0x1c)#define GPFCLR0        *(volatile uint32_t *)(GPIO_BASE + 0x28)#define GPFSET1        *(volatile uint32_t *)(GPIO_BASE + 0x20)#define GPFCLR1        *(volatile uint32_t *)(GPIO_BASE + 0x2c)void pinFunc(unsigned int pinN, uint32_t funcSet){  // function to bits:    // setToInput: 0b000    // setToOutput: 0b001    // setToAltFuc0: 0b100    // setToAltFuc1: 0b101    // setToAltFuc2: 0b110    // setToAltFuc3: 0b111    // setToAltFuc4: 0b011    // setToAltFuc5: 0b010  // pin function group regesters (each group of 3 of the 30-some bits of the regester corrispond to the function set for each pin)    // pins 0-9: GPFSEL0    // pins 10-19: GPFSEL1    // pins 20-29: GPFSEL2    // pins 30-39: GPFSEL3    // pins 40-49: GPFSEL4    // pins 50-53: GPFSEL5  unsigned int bitPos = 3*(pinN%10);  volatile uint32_t* GPFSEL = (volatile uint32_t *)(GPIO_BASE + ((pinN / 10) * 4));  // intager division  // declare a pointer with a * before the name at any point, refrence the target of the pointer and not just the pos by including the defferance operator, which is conincedentaly also an astrix before a pointer var  *GPFSEL &= ~(0b111 << bitPos);  // clears the position of the funcset area, by putting ...000000011100000000... at the pos, then inverting to only and (clear) the necessary 3 bits, and set everything else to 1, so that it preserves the original setting when anded.  *GPFSEL |= (funcSet << bitPos);  // gets the full 32 bit func like ...00000000"001"000000... then oring it to make sure not to clear anytning else by setting it to 0}// set/clear regesters for different pins  // pins 0-31: CLR0/SET0  // pins 32-53: CLR1/SET1void pinOn(unsigned int pinN){  // 1 leftshifted by the pin number into the regester  if (pinN <= 31){    GPFSET0 = (1 << pinN);  // assignes to the regester its pointing at. deference operator * not neccessary as it's baked into the definition at the top  }  else if (pinN >= 32){    GPFSET1 = (1 << (pinN-32));  }}void pinOff(unsigned int pinN){  if (pinN <= 31){    GPFCLR0 = (1 << pinN);  }  else if (pinN >= 32){    GPFCLR1 = (1 << (pinN-32));  }}void OSDelay(int reps){  while (reps--) {    asm volatile("nop"); // empty loop to create delay, compiler might optomize but I don't trust macos that much  }}void I2CInitalize() {  // Set the I2C clock rate, address, and enable I2C mode  I2C_C = 0;  // Disable I2C temporarily for configuration  I2C_A = 0x27;  // Address for your LCD, adjust as needed. only the 1st 7 bits of the 32 of this regester matter/are written to, it is the address. in this case, it's address 39, or 0b100111, or 0x27  // Set clock rate by configuring I2C_C (Clock Control)  // Enable the interface after configuration  I2C_C |= (1 << 15);  // Enable I2C. Bit 15 is the BSC (Broadcom (manufacturer of raspi2 processor) Serial Controller) enable/disable (1/0) (page 29)}void I2CByteSend(uint8_t cmdOrData){  I2C_DLEN = 1;  // DLEN has 16 bits to store the length. it counts through this to send the bits one at a time  I2C_FIFO = cmdOrData;  // there are some commands that I can send to the screen to do certain things. EG, 0x01 clears, 0x0C turns on w/o cursor, 0x06 set auto increment for cursor, 0x30 wake up (send multiple times), 0x28 turn on 4 bit mode and 2 line display   I2C_C |= (1 << 7);  // sets the start (7) bit to 1, to start transmission of command  while (!(I2C_S & (1 << 1)));  // checks if the DONE (1) bit, anded with 1 = 1. if it equals 0, then invertend and the loop continues, but if 1, inverted is 0, loop ends  I2C_S |= (1 << 1); // clear the DONE bit }/*the FIFO regester sends data to the LCD. the bits work out like this:bits 0:3 - Char/Commandbit 4 - Regester select (RS) (0 for sending command, 1 for sending char/data)bit 5 - read/write (R/W) (0 for writing to LCD, 1 for reading from LCD)bit 6 - enable (E), rising edge signal (0 -> 1), then falling edge (1 -> 0) (send command twice with same data, exept change enable from 1 to 0) latches then sends data to LCDbit 7 - backlight control (BL) (if appliccable). usualy 1 for backlight on, 0 for backlight off*/unsigned short backlight = 1;  // bool backlight = true;void LCDSendCmd(uint8_t cmd){    uint8_t highNybble = cmd >> 4;  // Extract MSB  uint8_t lowNybble = cmd & 0x0F;  // Extract LSB  uint8_t settings = 0b00000000 | (backlight << 7); // & 0b[BL][E][RS = 1][R/W = 0]0000, *sends* a *command*  // send first nibbyl  I2CByteSend(settings | highNybble | (1 << 6));  // send it by 1ing Enable bit  I2CByteSend(settings | highNybble | (0 << 6));  // latch it by 0ing Enable bit  // send second nibbyl  I2CByteSend(settings | lowNybble | (1 << 6));  I2CByteSend(settings | lowNybble | (0 << 6));}void LCDSendChar(char ch){    uint8_t highNybble = ch >> 4;  // Extract MSB  uint8_t lowNybble = ch & 0x0F;  // Extract LSB  uint8_t settings = 0b00010000 | (backlight << 7); // & 0b[BL][E][RS = 0][R/W = 0]0000, *sends* a *command*  //0b[backlight]0000000  // send first nibbyl  I2CByteSend(settings | highNybble | (1 << 6));  I2CByteSend(settings | highNybble | (0 << 6));  // send second nibbyl  I2CByteSend(settings | lowNybble | (1 << 6));  I2CByteSend(settings | lowNybble | (0 << 6));}void kernel_main() {  // Initialize I2C for LCD  I2CInitalize();  // Set GPIO pins 16, 20, and 21 to output  pinFunc(16, 0b001);  pinFunc(20, 0b001);  pinFunc(21, 0b001);  // Test GPIO by blinking LEDs  for (int i = 0; i < 5; i++) {    pinOn(16);  // Turn on red LED    OSDelay(500000);    pinOff(16); // Turn off red LED    pinOn(20);  // Turn on green LED    OSDelay(500000);    pinOff(20); // Turn off green LED    pinOn(21);  // Turn on blue LED    OSDelay(500000);    pinOff(21); // Turn off blue LED  }  // Test LCD by sending initialization commands  LCDSendCmd(0x30); // Wake up LCD (send multiple times if needed)  LCDSendCmd(0x30);  LCDSendCmd(0x30);  LCDSendCmd(0x30);  LCDSendCmd(0x30);  OSDelay(50000);   // Small delay  LCDSendCmd(0x28); // 4-bit mode, 2-line display  LCDSendCmd(0x0C); // Display ON, cursor OFF  LCDSendCmd(0x01); // Clear display  OSDelay(2000);    // Wait for clear to complete  LCDSendCmd(0x06); // Auto-increment cursor  // Send a test message to the LCD  const char *message = "Hello, World!";  for (size_t i = 0; message[i] != '\0'; i++) {    LCDSendChar(message[i]);  }  // Toggle backlight for testing  backlight = 0; // Turn backlight off  LCDSendCmd(0x0C); // Refresh display with new settings  OSDelay(500000);  backlight = 1; // Turn backlight on  LCDSendCmd(0x0C); // Refresh display with new settings}

Code:

.section ".text.boot".global _start_start:  mrc p15, #0, r1, c0, c0, #5     // read value from coprocessors to r1: https://developer.arm.com/documentation/den0042/a/ARM-Processor-modes-and-Registers/Registers/Coprocessor-15  and r1, r1, #3                  // isolates the first 2 bits of the cp15 Cache Level ID Register (CLIDR) which displays: 00 - no cache, 01 - instrucution-only cache, 01 - data-only cach, 11 - unified cache   cmp r1, #0  bne halt  mov sp, #0x8000                 // stack pointer, only place I can put variables without trying to murder the pi and kernal (cuz the pi starts booting from there)  ldr r4, =__bss_start  ldr r9, =__bss_end  mov r5, #0  mov r6, #0  mov r7, #0  mov r8, #0  b       2f1:  stmia r4!, {r5-r8}  // stmia = store multiple increment after. all regesters from r5 to r8 get stored to r4, and then the address is incremented by one to the next pos (-ia suffix), and that address is stored back into r4 (the ! after r4)  // what this does is store the 16 bytes of 0 stored in r5-r8 (as defined in line 15-18) into r4, 0s out the whole bss section (the section for uninitialized c variables)  // this makes sure that all uninitialized c variables are set to 0 at runtime, else an error gets thrown  // technicaly you could just use 1 regester to 0 out the whole bss section by looping, but that's less efficient then 4 at a time, 0ing out 4 bytes per loop instead of 162:  cmp r4, r9  // checks every time it loops to function 1, only not branching back to 1 when the r4 address is at the r9 address (when all of the bss regesters have been set to 0 - when bss start = bss end - when the pointer for bss start reaches the address of bss end?)  blo 1b  // branch if (unsigned) less than (r4 than r9)  ldr r3, =kernel_main  // gets address of kernel main function from c. the = means address, kinda like & in c funciton param declaration  blx r3  // branches to address in r3 (kernal_main script)  //b halt  // not neccessary as it would go there anyways, but helps in this case to make absolutly sure that no undefined actions happen// this is what the unused cores branch to, and what core 0 branches to once kernel_main returnshalt:    wfe  // do nothing at low power mode  b halt  // loop

Code:

ENTRY(_start)SECTIONS{  /* this is copied from https://jsandler18.github.io/tutorial/boot.html */    /* Starts at LOADER_ADDR. */    . = 0x8000;    __start = .;    __text_start = .;    .text :    {        KEEP(*(.text.boot))        *(.text)    }    . = ALIGN(4096); /* align to page size */    __text_end = .;     __rodata_start = .;    .rodata :    {        *(.rodata)    }    . = ALIGN(4096); /* align to page size */    __rodata_end = .;     __data_start = .;    .data :    {        *(.data)    }    . = ALIGN(4096); /* align to page size */    __data_end = .;     __bss_start = .;    .bss :    {        bss = .;        *(.bss)    }    . = ALIGN(4096); /* align to page size */    __bss_end = .;    __end = .;}

Code:

# this is mostly copied from https://jsandler18.github.io/tutorial/boot.html, though i did need to change the command to fit mac and change the files arounddefault:  arm-none-eabi-gcc -mcpu=cortex-a7 -fpic -ffreestanding -c boot.S -o objects/boot.o  arm-none-eabi-gcc -mcpu=cortex-a7 -fpic -ffreestanding -std=gnu99 -c kernel.c -o objects/kernel.o -O2 -Wall -Wextra  arm-none-eabi-gcc -T linker.ld -o objects/myos.elf -ffreestanding -O2 -nostdlib objects/boot.o objects/kernel.o  arm-none-eabi-objcopy -O binary objects/myos.elf kernel7.img