My os currently has a boot loader and small kernel which the boot loader loads into conventional memory in the first MB of RAM. How do I map/find out how much memory my os can play with?

Let's say I wanted to load the next part of the kernel into memory above the first MB; What is the 'usual' method to detect whether that memory exists?

What I use is Interrupt 15h E820h: http://www.uruk.org/orig-grub/mem64mb.html

You can structure your map based on the information within it the most easiest from my experience.

Thanks

Is there any example FASM code that is a good starting point? I've had a very quick play with that interrupt and it keeps returning 64 from a base address of zero.

Search for E820, there are a lot of hits. Here's one I did:

http://board.flatassembler.net/topic.php?t=6217

Smiddy

Something to be aware of is E820 is not complete - some legacy regions (BIOS/VGA), MMIO and other ROM locations are not included. Looking at my code, I have a mix of hard-coded legacy regions [as listed in those links] following by the E820 map and finally the PCI enumeration (some are duplicates of the legacy and E820 but this should take precedence - though I've never encountered a case where these were different).