Init SS and SP by default

README.md

@@ -22,6 +22,7 @@ Hello world programs that run without an operating system.
         1.  [keyboard](bios_keyboard.S)
     1.  [Not testable in userland](not-testable-in-userland.md)
         1.  [Segment registers real mode](segment_registers_real_mode.S)
+            1 [SS (TODO)](ss.S)
     1.  APM
         1.  [APM shutdown](apm_shutdown.S)
         1.  [APM shutdown 2](apm_shutdown2.S)

TODO.md

@@ -17,6 +17,8 @@
 
     - http://reverseengineering.stackexchange.com/questions/2006/how-are-the-segment-registers-fs-gs-cs-ss-ds-es-used-in-linux
     - http://stackoverflow.com/questions/10810203/what-is-the-fs-gs-register-intended-for
+    - http://stackoverflow.com/questions/12760109/data-segment-in-x86-programs
+    - http://stackoverflow.com/questions/14480579/when-does-segment-registers-change
     - http://stackoverflow.com/questions/15335003/x86-protected-mode-segment-registers-purpose
     - http://stackoverflow.com/questions/17210620/assembler-calculating-a-memory-address-with-register-base?lq=1
     - http://stackoverflow.com/questions/20717890/how-to-interpret-gs0x14?lq=1
@@ -29,7 +31,6 @@
     - http://stackoverflow.com/questions/6611346/amd64-fs-gs-registers-in-linux
     - http://stackoverflow.com/questions/7844963/how-to-interpret-segment-register-accesses-on-x86-64?lq=1
     - http://stackoverflow.com/questions/928082/why-does-the-mov-instruction-have-to-be-used-this-way?lq=1
-    - http://stackoverflow.com/questions/14480579/when-does-segment-registers-change
 
     64-bit:
 

common.h

@@ -8,9 +8,16 @@ The big ones do bloat the executable.
 
 #define BEGIN \
     .code16;\
-    cli;\
-    xor %ax, %ax;\
-    mov %ax, %ds
+    cli ;\
+    xor %ax, %ax ;\
+    /* We must zero %ds for any data access.. */ \
+    mov %ax, %ds ;\
+    /* TODO What to move into BP and SP? http://stackoverflow.com/questions/10598802/which-value-should-be-used-for-sp-for-booting-process */ \
+    mov 0x0000, %bp ;\
+    /* Disables interrupts until the end of the next instruction. */ \
+    mov %ax, %ss ;\
+    /* We should set SP because BIOS calls may depend on that. TODO confirm. */ \
+    mov %bp, %sp
 
 #define END
 

ss.S

@@ -0,0 +1,11 @@
+/*
+TODO implement.
+
+SS register. I think the major effect of it is that anything that uses
+`SP` like `PUSH` and `POP`, will actually use `SS:SP`.
+*/
+
+#include "common.h"
+BEGIN
+hlt
+END