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| author | Robert Morris <rtm@nephron.lcs.mit.edu> | 2010-08-05 21:16:55 -0400 |
|---|---|---|
| committer | Robert Morris <rtm@nephron.lcs.mit.edu> | 2010-08-05 21:16:55 -0400 |
| commit | 1afc9d3fcaa7c5992659bb8b69f639b746dda2bc (patch) | |
| tree | e60a41707282ba0ef2c1e00b9cba97bb71339284 /vm.c | |
| parent | c99599784e950169d85bf1e4446e7dbfb1a40f59 (diff) | |
| download | xv6-labs-1afc9d3fcaa7c5992659bb8b69f639b746dda2bc.tar.gz xv6-labs-1afc9d3fcaa7c5992659bb8b69f639b746dda2bc.tar.bz2 xv6-labs-1afc9d3fcaa7c5992659bb8b69f639b746dda2bc.zip | |
add some comments
find out the hard way why user and kernel must have separate segment descriptors
Diffstat (limited to 'vm.c')
| -rw-r--r-- | vm.c | 9 |
1 files changed, 6 insertions, 3 deletions
@@ -93,12 +93,15 @@ ksegment(void) { struct cpu *c; - // Map once virtual addresses to linear addresses using identity map + // Map virtual addresses to linear addresses using identity map. + // Cannot share a CODE descriptor for both kernel and user + // because it would have to have DPL_USR, but the CPU forbids + // an interrupt from CPL=0 to DPL=3. c = &cpus[cpunum()]; c->gdt[SEG_KCODE] = SEG(STA_X|STA_R, 0, 0xffffffff, 0); c->gdt[SEG_KDATA] = SEG(STA_W, 0, 0xffffffff, 0); - c->gdt[SEG_UCODE] = SEG(STA_X|STA_R, 0x0, 0xffffffff, DPL_USER); - c->gdt[SEG_UDATA] = SEG(STA_W, 0x0, 0xffffffff, DPL_USER); + 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 c->gdt[SEG_KCPU] = SEG(STA_W, &c->cpu, 8, 0); |