User:Nedbrek/mmap

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In the constrained environment of assembly language programming, it is important to have a clear picture of where everything is in memory:

Range Area
0_0000..0_03FF Real mode IDT (reclaimable)
0_0400..0_04FF BIOS low data (reclaimable)
0_0500..0_7BFF boot stack (30,464 bytes)
0_7C00..0_7DFF boot sector (512 bytes)
0_7E00..3_FFFF available
4_0000..4_FCFF boot output block (BOB)
4_FD00..4_FFFF video config scratchpad (reclaimable)
5_0000..5_FFFF GDT+IDT
6_0000..6_FFFF stage 1 page tables
7_0000..7_FFFF output of BIOS memory map (INT 15/E820)
8_0000..9_FBFF possible BIOS EDA
9_FC00..9_FFFF definite BIOS EDA
A_0000..F_FFFF ROM
struct Bob /// Boot Output Block
{
   DWord bootDisk;  // drive letter stored in DX by the BIOS
   DWord vgaWidth;  // (pixels)
   DWord vgaHeight; // (pixels)
   DWord vgaBpp;    // bits per pixel
   DWord vgaLfbp;   // address of the linear frame buffer
   DWord vgaCaps;   // mode attributes (offset 0 of the get mode structure)
   DWord vgaMode;   // value passed to INT 10/4F01 (BIOS set video mode)
};

Memory is grouped into 64KB segments (to take advantage of an assembler trick for zeroing it).

Bootl delivers us directly to 64 bit mode, thus each PTE is 8 bytes. That allows for 512 PTEs per 4KB page, with 16 pages in the one segment allocated for the task.

They are:

Number Description
0 PML4 (root page table, VA[47:39])
1 PDP (page directory pointers, VA[38:30])
2 PDE (page directory entry, VA[29:21] - good for 2M pages positioned from 0 to 1GB)
3-15 available
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