New bootloader concept: sys-like util to generate a boot sector
Right now only floppies are supported
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19f598251d
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4 changed files with 224 additions and 29 deletions
42
host/fix-rom.c
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42
host/fix-rom.c
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/* QEMU and x86 hardware with PCI support (and up) require that BIOS options
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* rom have a valid checksum, that the sum of all their bytes equals zero.
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* This script edits a rom binary in-place so it fits that criteria.
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*/
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#include <stdio.h>
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#include <sys/types.h>
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#include <sys/stat.h>
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#include <unistd.h>
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#include <stdlib.h>
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#include <stdint.h>
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struct stat sbuf;
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int main(int argc, char** argv) {
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FILE* fd = fopen(argv[1], "r+");
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fstat(fileno(fd), &sbuf);
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if (sbuf.st_size & 0x1F) {
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fprintf(stderr, "Filesize is not a multiple of 512 bytes\n");
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exit(1);
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}
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// Fill out filesize flag
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fseek(fd, 2, SEEK_SET);
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fputc(sbuf.st_size >> 9, fd);
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// Calculate checksum
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fseek(fd, 0, SEEK_SET);
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off_t i;
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uint8_t s;
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for (i=0; i<sbuf.st_size; i++) {
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s+=fgetc(fd);
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}
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// Edit last byte so that checksum is 0
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fseek(fd, -1, SEEK_END);
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s=fgetc(fd)-s;
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fseek(fd, -1, SEEK_END);
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fputc(s, fd);
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fclose(fd);
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}
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202
host/sys.c
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202
host/sys.c
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#include <stdio.h>
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#include <string.h>
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#include <stdlib.h>
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#include <stdint.h>
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#define min(a, b) (((a) < (b)) ? (a) : (b))
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// argv[1] must be floppy image
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// argv[2] must be name of boot com
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// if you expect to see memory safety, close your editor NOW!
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#define AH 4
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#define AX 0
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#define CX 1
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#define DH 6
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#define BX 3
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#define SP 4
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#define SI 6
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#define DI 7
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char* push_imm8(char* addr, int i) {
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*(int8_t*)addr = i;
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return(addr+1);
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}
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char* push_imm16(char *addr, int i) {
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*(int16_t*)addr = i;
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return(addr+2);
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}
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#define push_mov8(a, b, c) push_imm8(push_imm8(a, 0xB0+b), c);
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char* push_mov16(char* a, int b, int c) {
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if (c) {
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return(push_imm16(push_imm8(a, 0xB8+b), c));
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} else {
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// if value is 0 for 16-bit reg, xor it with itself
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// saves a byte
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return(push_imm16(a, 0xC031 | (b << 11) | (b << 8)));
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}
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}
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char* push_jc(char* addr, char* target) {
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int diff = target - addr;
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return(push_imm8(push_imm8(addr, 0x72), diff-2));
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}
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char* push_jmp(char* addr, char* target) {
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int diff = target - addr;
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if (abs(diff)<0x70) {
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return(push_imm8(push_imm8(addr, 0xEB), diff-2));
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} else {
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return(push_imm16(push_imm8(addr, 0xE9), diff-3));
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}
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}
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int main(int argc, char** argv) {
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// query mshowfat for first and last cluster number
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// we expect the file to be contiguous
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char output[64], cmd[256]= "mshowfat -i ";
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strcat(cmd, argv[1]);
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strcat(cmd, " '::");
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strcat(cmd, argv[2]);
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strcat(cmd, "'");
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FILE* fd = popen(cmd, "r");
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fgets(output, 64, fd);
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int r = pclose(fd);
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if (r > 0) {
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perror("Unable to query mtools");
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exit(1);
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}
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// parse the two numbers out of the output
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// output is like "::kernel.bs <2-4>"
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char* p = strchr(output, '<');
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p++;
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int min = strtol(p,&p,10);
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int max = 0;
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if (*p=='-') {
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p++;
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max = strtol(p,&p,10);
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if (*p!='>') {
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perror("Unexpected result from mshowfat");
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exit(1);
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}
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} else {
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max = min;
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}
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if (*p!='>') {
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perror("Unexpected result from mshowfat");
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exit(1);
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}
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fd = fopen(argv[1], "r+");
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if (fd < 0) {
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perror("Cant open disk image");
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}
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char bootsect[512];
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if (fread(&bootsect, 512, 1, fd) < 1) {
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perror("Failed to read bootsector");
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exit(1);
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}
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// unpack fields from bpb
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int ss = *((uint16_t*)(bootsect+0x0B));
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int cs = *((uint8_t*)(bootsect+0x0D));
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int rsc = *((uint16_t*)(bootsect+0x0E));
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int fn = *((uint8_t*)(bootsect+0x10));
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int rde = *((uint16_t*)(bootsect+0x11));
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int fs = *((uint16_t*)(bootsect+0x16));
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int spt = *((uint16_t*)(bootsect+0x18));
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int nos = *((uint16_t*)(bootsect+0x1A));
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int base = rsc + (fs * fn) + ((rde * 32) / ss);
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int min_s = base + (min - 2) * cs;
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int max_s = base + (max - 2) * cs + (cs - 1);
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int bx = 0x100;
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int es = 0x70;
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char* bs = (char*)&bootsect;
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char* ptr = bs+0x3E;
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char* putsfunc = ptr;
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// assemble puts function
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ptr = push_imm16(ptr, 0xAC5E);
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ptr = push_imm16(ptr, 0xC084);
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ptr = push_imm16(ptr, 0x0874);
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ptr = push_imm16(ptr, 0x0EB4);
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ptr = push_imm16(ptr, 0xDB31);
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ptr = push_imm16(ptr, 0x10CD);
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ptr = push_imm16(ptr, 0xF3EB);
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ptr = push_imm16(ptr, 0xE6FF);
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// assemble entry point to our main code
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push_jmp(bs, ptr);
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// assemble segment and stack setup
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ptr = push_mov16(ptr, AX, es);
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ptr = push_imm16(ptr, 0xD08E); // mov sp, ax
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ptr = push_mov16(ptr, SP, 0);
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// save two copies of our working segment on the stack
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ptr = push_imm8(ptr, 0x50 + AX); // push ax
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ptr = push_imm8(ptr, 0x50 + AX); // push ax
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// assemble relocation code
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ptr = push_mov16(ptr, AX, 0);
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ptr = push_imm16(ptr, 0xD88E); // mov ds, ax
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ptr = push_mov16(ptr, SI, 0x7C00);
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ptr = push_imm16(ptr, 0xC08E); // mov es, ax
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ptr = push_mov16(ptr, DI, (es-0x10) << 4);
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ptr = push_mov16(ptr, CX, 0x0100);
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ptr = push_imm16(ptr, 0xA5F3); // rep movsw (does the copy)
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ptr = push_imm8(ptr, 0xEA); // jump far
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ptr = push_imm16(ptr, ptr+4-bs+0x600);
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ptr = push_imm16(ptr, 0);
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// pop DS and ES from the stack
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ptr = push_imm8(ptr, 0x07); // pop es
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ptr = push_imm8(ptr, 0x1F); // pop ds
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while(min_s <= max_s) {
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int sector = (min_s % spt) + 1;
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int tmp = min_s / spt;
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int head = tmp % nos;
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int track = tmp / nos;
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int len = min(spt - sector, max_s - min_s)+1;
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// memorize jump address in case of error
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char* retry = ptr;
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// set up registers for drive reset
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ptr = push_imm8(push_imm8(ptr, 0xB0+AH), 0);
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ptr = push_imm16(ptr, 0x13CD);
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// set up registers for read call
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ptr = push_imm16(push_imm8(ptr, 0xB8+AX), 0x0200 | len);
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ptr = push_imm16(push_imm8(ptr, 0xB8+CX), sector | (track << 8));
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ptr = push_imm8(push_imm8(ptr, 0xB0+DH), head);
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ptr = push_imm16(push_imm8(ptr, 0xB8+BX), bx);
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// set up read call
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ptr = push_imm16(ptr, 0x13CD);
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// jump if carry to retry addr
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ptr = push_jc(ptr, retry);
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min_s+=len;
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bx=bx+(ss*len);
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}
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// emerg return addr
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ptr = push_mov16(ptr, AX, 0);
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ptr = push_imm8(ptr, 0x50 + AX);
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ptr = push_imm8(ptr, 0xEA); // jmp far
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ptr = push_imm16(ptr, 0x100);
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ptr = push_imm16(ptr, es);
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// emerg return int 0
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ptr = bs + 0x100;
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ptr = push_imm16(ptr, 0x18CD);
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// write back bootsector
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fseek(fd, 0, SEEK_SET);
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if (fwrite(&bootsect, 512, 1, fd) < 1) {
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perror("Failed to write back boot sector");
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exit(1);
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}
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}
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