1 files changed, 97 insertions, 0 deletions
diff --git a/x86-bare-metal/mbr-e820/mbr.zig b/x86-bare-metal/mbr-e820/mbr.zig
new file mode 100644
index 0000000..3b6f71d
--- /dev/null
+++ b/x86-bare-metal/mbr-e820/mbr.zig
@@ -0,0 +1,97 @@
+const bios = @import("bios.zig");
+
+// -- ENTRY POINT ---------------------------------------------------------------
+
+export fn _entry() linksection(".boot") callconv(.Naked) noreturn {
+    asm volatile (
+        // Disable interrupts.
+        \\cli
+        // Clear segment selectors.
+        \\xor %%ax, %%ax
+        \\mov %%ax, %%ds
+        \\mov %%ax, %%es
+        \\mov %%ax, %%ss
+        \\mov %%ax, %%fs
+        \\mov %%ax, %%gs
+        \\mov $0x7c00, %sp
+        // Long jump to set cs to 0x0000, as some BIOSes load the MBR
+        // to either 07c0:0000 or 0000:7c000.
+        \\ljmp $0x0, $main
+    );
+}
+
+// -- MAIN ----------------------------------------------------------------------
+
+// main should be "callconv(.naked)", once issue is fixed.
+// https://github.com/ziglang/zig/issues/18183
+export fn main() noreturn {
+    dump_memmap();
+
+    while (true) {
+        asm volatile ("hlt");
+    }
+}
+
+// -- UTILS ---------------------------------------------------------------------
+
+fn dump_memmap() void {
+    // Keep track of continuatation code.
+    var off: u32 = 0;
+    // Accumulated free/usable memory.
+    var mem: u64 = 0;
+
+    // Iterate over E820 entries.
+    while (bios.e820(off)) |entry| : (off = entry.ebx) {
+        puts_hex(entry.e820.base);
+        bios.putc('-');
+        puts_hex(entry.e820.size);
+        bios.putc('-');
+        puts_hex(entry.e820.kind);
+        bios.putc('-');
+        puts_hex(entry.e820.attr);
+        bios.putc('\n');
+
+        // Accumulate free, useable memory.
+        if (entry.e820.kind == 1 and entry.e820.attr == 1) {
+            mem += entry.e820.size;
+        }
+
+        // All E820 entries retrieved, continuing would start from the beginning.
+        if (entry.ebx == 0) {
+            break;
+        }
+    }
+
+    // Report total free memory in MBs.
+    bios.putc('m');
+    bios.putc('=');
+    puts_hex(mem / 1024 / 1024);
+}
+
+fn puts(str: []const u8) void {
+    for (str) |c| {
+        bios.putc(c);
+    }
+}
+
+fn puts_hex(init_val: u64) void {
+    if (init_val == 0) {
+        bios.putc('0');
+        return;
+    }
+
+    var val = init_val;
+    var buf: [32]u8 = undefined;
+    var idx = buf.len;
+
+    while (val > 0 and idx != 0) : (val /= 16) {
+        idx -= 1;
+        const digit = switch (@as(u8, @truncate(val % 16))) {
+            0...9 => |d| '0' + d,
+            10...15 => |d| 'a' + d - 10,
+            else => '?',
+        };
+        buf[idx] = digit;
+    }
+    puts(buf[idx..]);
+}