fix: identity-map first 4 GiB to cover framebuffer MMIO

Co-authored-by: aider (openrouter/anthropic/claude-opus-4.7) <aider@aider.chat>

kernel_entry.S

@@ -34,7 +34,7 @@ pml4:
 pdpt:
     .space 4096
 pd:
-    .space 4096
+    .space 4096 * 4
 
 .align 16
 stack_bottom:
@@ -64,13 +64,22 @@ _start:
     orl $0x03, %eax
     movl %eax, pml4
 
+    /* Populate 4 PDPT entries, each pointing to one of 4 PDs.
+     * This identity-maps the first 4 GiB using 2 MiB pages,
+     * which covers typical framebuffer/MMIO regions. */
     movl $pd, %eax
     orl $0x03, %eax
-    movl %eax, pdpt
+    movl %eax, pdpt + 0
+    addl $4096, %eax
+    movl %eax, pdpt + 8
+    addl $4096, %eax
+    movl %eax, pdpt + 16
+    addl $4096, %eax
+    movl %eax, pdpt + 24
 
     movl $0x00000083, %eax
     movl $pd, %edi
-    movl $512, %ecx
+    movl $2048, %ecx
 1:
     movl %eax, (%edi)
     addl $0x200000, %eax