memory map

Memory Map

Every physical address MyOS-Simple touches, from the interrupt vector table to the top of video memory.

MyOS-Simple never enables paging, so every address in this reference is a physical address — what the CPU puts on the bus is exactly what the RAM chip (or the VGA card) sees. There is no virtual memory, no page tables, and no MMU translation. The flat memory model means a pointer in C and a physical address are the same number.

This page is the authoritative layout for all C stages (2–5). The plan is identical across them; only the kernel's size (and therefore the sector count loaded by the bootloader) changes.

The map at a glance

 physical address
 0x00000  +-------------------------------+
          | Real-mode IVT + BIOS data     |   256 vectors x 4 bytes, then BDA
 0x00500  +-------------------------------+
          | (free low memory)             |
 0x01000  +-------------------------------+  <- KERNEL LOAD ADDRESS
          | Kernel image                  |     linker places .text here;
          |   .text .rodata .data .bss    |     execution starts at the first byte
          |   ...                         |
 0x07C00  +-------------------------------+  <- BOOT SECTOR (loaded by BIOS)
          | Boot sector (512 B, 0xAA55)   |     512 bytes, runs in real mode
 0x07E00  +-------------------------------+
          | (free)                        |
 0x90000  +-------------------------------+  <- PROTECTED-MODE STACK TOP
          | Stack (grows DOWNWARD)        |     ESP = EBP = 0x90000
          |              v v v            |
          +-------------------------------+
 0xB8000  +-------------------------------+  <- VGA TEXT FRAMEBUFFER (MMIO)
          | Video memory: 80x25 cells     |     each cell = char byte + attr byte
 0xC0000  +-------------------------------+  <- end of region of interest

Region table

Start	End	Region	Contents	Set up by	Notes
0x00000	0x004FF	IVT + BIOS data area	256 real-mode interrupt vectors (4 bytes each) followed by the BIOS Data Area	Firmware	Unused once in protected mode; the IVT no longer drives interrupts
0x00500	0x00FFF	Free low memory	Unused conventional RAM	—	Available scratch space
0x01000	~0x07BFF	Kernel image	.text, .rodata, .data, .bss	linker.ld + boot.asm disk load	KERNEL_OFFSET equ 0x1000 in boot.asm:15; execution begins at the first byte
0x07C00	0x07DFF	Boot sector	The 512-byte boot sector, ending in 0xAA55	BIOS (loads it here)	[ORG 0x7C00] in boot.asm:13; runs in real mode
0x07E00	0x8FFFF	Free	Unused	—	The stack lives at the top of this span
0x90000	(downward)	Stack	Protected-mode call stack	boot.asm:57-58	ESP = EBP = 0x90000, grows toward lower addresses
0xB8000	0xBFFFF	VGA text framebuffer	80×25 character cells, MMIO	Hardware	Each cell is a char byte + an attribute byte
0xC0000	—	End of interest	(Video BIOS / option ROMs beyond)	—	Outside what MyOS-Simple uses

Kernel sections (from 0x1000 upward)

The linker script lays the freestanding kernel out in this order, starting at the load address:

Order	Section	Holds	Initialized?
1	.text	Executable code	Yes (in image)
2	.rodata	String literals, constant tables (e.g. the scancode tables)	Yes (in image)
3	.data	Initialized globals	Yes (in image)
4	.bss	Zero-initialized globals	No (not in image)

Because .text is placed first at 0x1000, the bootloader can simply call 0x1000 (boot.asm:61) and land on the kernel's entry stub.

Why these addresses

Address	Rationale
0x7C00	Firmware convention since the original IBM PC. It equals 32768 − 1024: the last kilobyte of the first 32 KiB of RAM, leaving the low space free for the OS.
0x1000	Comfortably above the IVT/BDA and free low memory, and well below the boot sector at 0x7C00, so the loaded kernel never overlaps the still-running bootloader.
0x90000	High enough to give the stack room to grow downward without colliding with the kernel, yet below the 0xA0000 legacy video/BIOS region.
0xB8000	Fixed hardware address of the VGA text-mode framebuffer; it is not RAM you chose but memory-mapped device memory.

💡 Tidbit: 0x7C00 is 32768 − 1024. The original IBM PC shipped with as little as 32 KiB of RAM, and the designers loaded the boot sector into the last kilobyte of that range so the freshly booted code had the maximum contiguous block of low memory beneath it.

⚠️ Caveat: The stack and the kernel image are not separated by any guard. ESP starts at 0x90000 and grows down; the kernel image grows up from 0x1000. A runaway recursion or a huge local buffer can silently walk the stack down into free memory — there is no page fault to catch it because paging is off.

💡 Tidbit: truncate -s 10240 pads stage 2/3 images to exactly 20 sectors (20 × 512 = 10240); stages 4 and 5 use truncate -s 20480 (40 sectors). The pad only guarantees the file is large enough for the bootloader's CHS read — it does not affect the in-memory map above. See toolchain-and-build.md.

See also

• Boot process — how control reaches each region
• Boot sector — the 0x7C00 payload
• Linker scripts — how .text is placed at 0x1000
• Protected mode — the flat model that makes these physical addresses
• VGA text mode — the 0xB8000 framebuffer
• I/O ports — the device side of the map
• Glossary
• Home