tiny clarifications to some gdt code.

858475e4 · Robert Morris · 0a4a4230 · 858475e4 · 858475e4 · 858475e4
--- a/mmu.h
+++ b/mmu.h
@@ -39,6 +39,7 @@

 #define CR4_PSE         0x00000010      // Page size extension

+// various segment selectors.
 #define SEG_KCODE 1  // kernel code
 #define SEG_KDATA 2  // kernel data+stack
 #define SEG_KCPU  3  // kernel per-cpu data
@@ -46,6 +47,9 @@
 #define SEG_UDATA 5  // user data+stack
 #define SEG_TSS   6  // this process's task state

+// cpu->gdt[NSEGS] holds the above segments.
+#define NSEGS     7
+
 //PAGEBREAK!
 #ifndef __ASSEMBLER__
 // Segment Descriptor

--- a/proc.h
+++ b/proc.h
-// Segments in proc->gdt.
-#define NSEGS     7
-
 // Per-CPU state
 struct cpu {
  uchar id;                    // Local APIC ID; index into cpus[] below

--- a/vm.c
+++ b/vm.c
@@ -9,7 +9,6 @@

 extern char data[];  // defined by kernel.ld
 pde_t *kpgdir;  // for use in scheduler()
-struct segdesc gdt[NSEGS];

 // Set up CPU's kernel segment descriptors.
 // Run once on entry on each CPU.
@@ -28,7 +27,7 @@ seginit(void)
  c->gdt[SEG_UCODE] = SEG(STA_X|STA_R, 0, 0xffffffff, DPL_USER);
  c->gdt[SEG_UDATA] = SEG(STA_W, 0, 0xffffffff, DPL_USER);

-  // Map cpu, and curproc
+  // Map cpu and curproc -- these are private per cpu.
  c->gdt[SEG_KCPU] = SEG(STA_W, &c->cpu, 8, 0);

  lgdt(c->gdt, sizeof(c->gdt));