Changed to nasm

1 files changed, 36 insertions, 0 deletions

diff --git a/src/gdt.c b/src/gdt.c
new file mode 100644
index 0000000..a9808aa
--- /dev/null
+++ b/src/gdt.c
@@ -0,0 +1,36 @@
+#include "gdt.h"
+
+void encodeGDT(uint8_t* gdtEntry, struct GDT source) {
+    if ((source.limit > 65536) && ((source.limit & 0xFFF) == 0xFFF)) {
+        // Set the GDT to use paging
+        // To do this we need to make sure the limit is aligned to 4KiB
+        source.limit = source.limit >> 12;
+        // Granularity: 1 (use paging with 4KiB segments)
+        // Size: 1 (32 bit protected mode)
+        gdtEntry[6] = 0xC0;
+    }
+    else {
+        // Granularity: 0 (1 byte segments)
+        // Size: 1
+        gdtEntry[6] = 0x40;
+    }
+    
+    // Here we are encoding the limit
+
+    // Bits 0-15 encode the bottom 16 bits of the limit
+    gdtEntry[0] = source.limit & 0xFF;
+    gdtEntry[1] = (source.limit >> 8) & 0xFF;
+    // Bits 48-51 encode the last 4 bits of the limit
+    gdtEntry[6] |= (source.limit >> 16) & 0xF;
+
+    // Bits 16-39 encode the bottom 24 bits of the base
+    gdtEntry[2] = source.base & 0xFF;
+    gdtEntry[3] = (source.base >> 8) & 0xFF;
+    gdtEntry[4] = (source.base >> 16) & 0xFF;
+    // Bits 56-64 encode the last byte of the base
+    gdtEntry[7] = (source.base >> 24) & 0xFF;
+
+    // Bits 40-47 set the access byte, which is documented at https://wiki.osdev.org/GDT,
+    // where most of the ideas for this function are taken from shamelessly
+    gdtEntry[5] = source.type;
+}