xv6: formatting, cleanup, rev5 (take 2)

1 files changed, 21 insertions, 2 deletions

diff --git a/data.S b/data.S
index 47f05b3..c0eb55b 100644
--- a/data.S
+++ b/data.S
@@ -1,5 +1,24 @@
-# Define "data" symbol to mark beginning of data segment.
-# Must be linked before any other data on ld command line.
+// The kernel layout is:
+//
+//     text
+//     rodata
+//     data
+//     bss
+//
+// Conventionally, Unix linkers provide pseudo-symbols
+// etext, edata, and end, at the end of the text, data, and bss.
+// For the kernel mapping, we need the address at the beginning
+// of the data section, but that's not one of the conventional
+// symbols, because the convention started before there was a
+// read-only rodata section between text and data.
+//
+// To get the address of the data section, we define a symbol
+// named data and make sure this is the first object passed to
+// the linker, so that it will be the first symbol in the data section.
+//
+// Alternative approaches would be to parse our own ELF header
+// or to write a linker script, but this is simplest.
+
 .data
 .globl data
 data: