User:Nedbrek/mmap

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