Rolling Your Own Bootloader: Difference between revisions

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 === Where will you load your kernel ? ===
+You will have to decide where in memory you are going to load your kernel. Your kernel generally depends on it.
-That's one of the first questions you need to answer. Being in [[Real Mode]], the easiest is to stay below the 1MB barrier -- which means you practically have 512KB of memory to load things. You may wish the kernel to be loaded at a well-known position -- say 0x10000 physical (es=0x1000, bx=0 when calling INT13h).
-; tip: If your kernel is bigger (or is expected to grow bigger) than this, you'll probably prefer to have the kernel above the 1MB barrier, which means you need to activate [[A20 Line|A20]] gate and switch to [[Unreal Mode]] to load the kernel (with A20 alone, you cannot have more than 64K above 1MB).
+In RealMode, the easiest is to stay below the 1MB barrier, which means you practically have 512KB of memory to load things. You may wish the kernel to be loaded at a well-known position, say 0x10000 physical (es=0x1000, bx=0 when calling INT13h).
-; caveat: Note that BIOS will still be unable to write to memory above 1MB, so you need to read stuff in a buffer below 1MB and ''then'' manually copy those stuff above 1MB.
+If your kernel is bigger (or is expected to grow bigger) than this, you'll probably prefer to have the kernel above the 1MB barrier, which means you need to activate [[A20 Line|A20 gate]] and switch to [[Unreal Mode]] to load the kernel (with A20 alone, you cannot have more than 64K above 1MB).
+Note that BIOS will still be unable to write to memory above 1MB, so you need to read stuff in a buffer below 1MB and then perform a rep movsd to place the data where they ultimately should go.
 === How will you find your kernel ? ===

Rolling Your Own Bootloader: Difference between revisions

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