1 files changed, 77 insertions, 0 deletions
diff --git a/x86-bare-metal/mbr-e820/bios.zig b/x86-bare-metal/mbr-e820/bios.zig
new file mode 100644
index 0000000..4479c82
--- /dev/null
+++ b/x86-bare-metal/mbr-e820/bios.zig
@@ -0,0 +1,77 @@
+/// BIOS starts with video mode 0h.
+///   * 80x25 text mode
+///   * 16 colors (4bit)
+/// http://www.ctyme.com/intr/rb-0069.htm
+pub const COLS = 80;
+pub const ROWS = 25;
+
+/// BIOS CALL - video teletype output.
+/// http://www.ctyme.com/intr/rb-0106.htm
+pub fn putc(c: u8) void {
+    // ah=0eh (call nr)
+    // al=c   (ascii char to write)
+    const ax = (@as(u16, 0xe) << 8) | c;
+
+    _ = asm volatile (
+        \\int $0x10
+        : // No outputs.
+        : [a] "{ax}" (ax), // BIOS call number + char to write.
+        : "cc"
+    );
+
+    // On linefeed also write a carriage return to move to column 0.
+    if (c == '\n') {
+        putc('\r');
+    }
+}
+
+const E820 = extern struct {
+    base: u64 = 0,
+    size: u64 = 0,
+    kind: u32 = 0,
+    attr: u32 = 1, // ACPI 3.0 compat
+};
+
+/// BIOS CALL - get system memory map
+/// http://www.ctyme.com/intr/rb-1741.htm
+/// https://wiki.osdev.org/Detecting_Memory_(x86)#BIOS_Function:_INT_0x15,_EAX_=_0xE820
+pub fn e820(next_ebx: u32) ?struct { e820: E820, ebx: u32 } {
+    var res = E820{};
+
+    // Setup call number.
+    var eax: u32 = 0xe820;
+    // Setup size of entry, bios call will return actual bytes written.
+    var ecx: u32 = @sizeOf(@TypeOf(res));
+    // Setup continuation code, next one will be returned from the bios call.
+    var ebx: u32 = next_ebx;
+
+    const carry = asm volatile (
+        \\int $0x15
+        \\setc %[c]
+        : // Outputs.
+          [n] "+{eax}" (eax),
+          [l] "+{ecx}" (ecx),
+          [o] "+{ebx}" (ebx),
+          [c] "={dl}" (-> u8), // Returns carry bit, recycle dl from dx.
+        : // Inputs.
+          [p] "{edi}" (&res), // Pointer to E820 entry.
+          [m] "{edx}" (0x534D4150), // Magic number (b"SMAP").
+        : // Clobbers.
+          "cc"
+    );
+
+    // Call failed if carry is set.
+    if (carry == 1) {
+        return null;
+    }
+    // Call failed if magic number is not returned in eax.
+    if (eax != 0x534D4150) {
+        return null;
+    }
+    // Call failed if partial entry is written.
+    if (ecx < 20) {
+        return null;
+    }
+
+    return .{ .e820 = res, .ebx = ebx };
+}