ram32: Better doucment 64-bit code descriptor

Signed-off-by: Joe Richey <joerichey@google.com>

Author: josephlr

src/asm/ram32.s

@@ -54,7 +54,18 @@ gdt64_ptr:
     .short gdt64_end - gdt64_start - 1 # GDT length is actually (length - 1)
     .long gdt64_start
 gdt64_start:
-    .quad 0                                     # First descriptor is null
+    # First descriptor is null
+    .quad 0
 code64_desc:
-    .quad (1<<43) | (1<<44) | (1<<47) | (1<<53) # Only these bits do anything
+    # For 64-bit code descriptors, all bits except the following are ignored:
+    # - CS.A=1 (bit 40) segment is accessed, prevents a write on first use.
+    # - CS.R=1 (bit 41) segment is readable. (this might not be necessary)
+    # - CS.C=1 (bit 42) segment is conforming. (this might not be necessary)
+    # - CS.E=1 (bit 43) required, we are a executable code segment.
+    # - CS.S=1 (bit 44) required, we are not a system segment.
+    # - CS.DPL=0 (bits 45/46) we are using this segment in Ring 0.
+    # - CS.P=1 (bit 47) required, the segment is present.
+    # - CS.L=1 (bit 53) required, we are a 64-bit (long mode) segment.
+    # - CS.D=0 (bit 54) required, CS.L=1 && CS.D=1 is resevered for future use.
+    .quad (1<<40) | (1<<41) | (1<<42) | (1<<43) | (1<<44) | (1<<47) | (1<<53)
 gdt64_end: