implement CHS and LBA conversion functions. seem to be working right

include/chs.h

@@ -3,6 +3,25 @@
 
 #include <stdint.h>
 
-void decode_chs(const uint8_t* chs, uint16_t* cylinder, uint16_t* head, uint16_t* sector);
+typedef struct {
+    uint8_t b[3];
+} __attribute__((packed)) chs_encoded_t;
+
+typedef struct {
+    uint16_t cylinder;
+    uint16_t head;
+    uint16_t sector;
+} chs_t;
+
+typedef struct {
+    uint16_t num_cylinders;
+    uint16_t num_heads;
+    uint16_t num_sectors;
+} chs_conf_t;
+
+void chs_decode(const chs_encoded_t* enc, chs_t* chs);
+
+uint32_t chs_to_lba(const chs_conf_t* conf, const chs_t* chs);
+void lba_to_chs(const chs_conf_t* conf, uint32_t lba, chs_t* chs);
 
 #endif

include/mbr.h

@@ -2,12 +2,13 @@
 #define __MBR_H
 
 #include <stdint.h>
+#include "chs.h"
 
 typedef struct {
     uint8_t attributes;
-    uint8_t chs_first[3];
+    chs_encoded_t chs_first;
     uint8_t type;
-    uint8_t chs_last[3];
+    chs_encoded_t chs_last;
     uint32_t lba_start;
     uint32_t num_sectors;
 } __attribute__((packed)) mbr_part_t;

src/chs.c

@@ -1,8 +1,20 @@
 #include "chs.h"
 
-void decode_chs(const uint8_t* chs, uint16_t* cylinder, uint16_t* head, uint16_t* sector)
+void chs_decode(const chs_encoded_t* enc, chs_t* chs)
 {
-    *head = chs[0];
-    *sector = chs[1] & 0x3F;
-    *cylinder = (((uint16_t)chs[1]) & 0xC0) << 2 | (uint16_t)chs[2];
+    chs->head = enc->b[0];
+    chs->sector = enc->b[1] & 0x3F;
+    chs->cylinder = (((uint16_t)enc->b[1]) & 0xC0) << 2 | (uint16_t)enc->b[2];
+}
+
+uint32_t chs_to_lba(const chs_conf_t* conf, const chs_t* chs)
+{
+    return (chs->cylinder * conf->num_heads + chs->head) * conf->num_sectors + (chs->sector - 1);
+}
+
+void lba_to_chs(const chs_conf_t* conf, uint32_t lba, chs_t* chs)
+{
+    chs->head = lba / (conf->num_heads * conf->num_sectors);
+    chs->cylinder = (lba / conf->num_sectors) % conf->num_heads;
+    chs->sector = (lba % conf->num_sectors) + 1;
 }

src/readmbr.c

@@ -37,17 +37,17 @@ int main(int argc, char** argv)
     for (unsigned i = 0; i < 4; i++)
     {
         const mbr_part_t* part = &mbr.part_table[i];
-        uint16_t cylinder, head, sector;
+        chs_t chs_first, chs_last;
+
+        chs_decode(&part->chs_first, &chs_first);
+        chs_decode(&part->chs_last, &chs_last);
 
         printf("Partition:\t%u\n", i);
         printf("\tAttributes:\t%x (%s)\n", part->attributes, part->attributes & 0x80 ? "active" : "");
         printf("\tType:\t%x\n", part->type);
 
-        decode_chs(part->chs_first, &cylinder, &head, &sector);
-        printf("\tCHS First:\tC %u\tH %u\tS %u\n", cylinder, head, sector);
-        
-        decode_chs(part->chs_last, &cylinder, &head, &sector);
-        printf("\tCHS Last:\tC %u\tH %u\tS %u\n", cylinder, head, sector);
+        printf("\tCHS First:\tC %u\tH %u\tS %u\n", chs_first.cylinder, chs_first.head, chs_first.sector);
+        printf("\tCHS Last:\tC %u\tH %u\tS %u\n", chs_last.cylinder, chs_last.head, chs_last.sector);
         
         printf("\tLBA:\t%u\n", part->lba_start);
         printf("\tTotal Sectors:\t%u\n", part->num_sectors);