UNIX for Dummies Questions & Answers

Need some clarification on this....

1. how are kernel/ user spaces and high/low memory related?

2. What do they all mean when i have the kernel command line as:
"console=ttyS0,115200 root=/dev/sda2 rw mem=exactmap memmap=1M@0 memmap=96M@1M irqpoll"
or
2. what do mem and memmap mean in terms of user/kernel space and high/low mem?

The traditional virtual mapping for the kernel in linux x86 is 1 gig of address space for the kernel, 3 gigs of address space for one process. There's also a 128-meg hole punched into the kernel range for hardware reasons, which limits the amount of memory the kernel can actually access to potentially less than the system has installed. Memory the kernel can't map into itself is "high memory".

It can use that 128-meg hole indirectly, at a performance cost.

How this matters to userspace is either less available memory, or a performance cost. Make /dev/console my serial port, defaulting to 115200 baud. This is handy for kernel debugging since you can log crashdumps that come over a serial port, but can't really log a monitor...

Code:

root=/dev/sda2 rw

Mount /dev/sda2 as my root partition in read-write mode

Code:

mem=exactmap memmap=1M@0 memmap=96M@1M

Ignore the used-memory ranges the BIOS reports and use these numbers instead. grepping for it finds it in arch/x86/kernel/e820.c Some badly broken BIOSes screw up interrupt handlers, giving the wrong interrupt numbers etc etc. Interrupts still happen but may call the wrong handlers, or no handlers. irqpoll is a workaround to check all available handlers to see if they're waiting for an interrupt and service it accordingly. I had a system that needed this, without irqpoll the network card would just cause kernel logs like "IRQ 12, nobody cared".

The difference to userspace is 128 less megs of memory. high memory is a kernel thing, in userspace the distinction is moot.

[edit] almost forgot. more detail here

how to find what are the current settings for high and low memory?

make menuconfig?