10.5.2. Running Linux

This test uses a Linux 2.6.19 kernel built for the standalone Or1ksim as described in Embecosm Application Note 2. The OpenCores OpenRISC 1000 Simulator and Tool Chain: Installation Guide. [4]. A configuration file, which enables the internal memory management units (MMUs) and Programmable Interrupt Controller (PIC) of the Or1ksim is provided, linux.cfg. This also declares additional internal memory space in Or1ksim for flash and SRAM.

The SystemC model is then run with this configuration file and the Linux kernel binary.

./IntrSocSC linux.cfg ../linux-2.6.19/vmlinux

Initially Linux copies itself from flash memory to RAM.

Copying Linux... Ok, booting the kernel.

After a pause while initial booting is taking place the serial interface is ready, allowing the normal kernel boot messages to appear:

Linux version 2.6.19-or32 (jeremy@thomas) (gcc version 3.4.4) #59 Wed Jun 25 18:
48:06 BST 2008
Detecting Processor units:
  Signed 0x391
Setting up paging and PTEs.
write protecting ro sections (0xc0002000 - 0xc024c000)
Setting up identical mapping (0x80000000 - 0x90000000)
Setting up identical mapping (0x92000000 - 0x92002000)
Setting up identical mapping (0xb8070000 - 0xb8072000)
Setting up identical mapping (0x97000000 - 0x97002000)
Setting up identical mapping (0x99000000 - 0x9a000000)
Setting up identical mapping (0x93000000 - 0x93002000)
Setting up identical mapping (0xa6000000 - 0xa6100000)
Setting up identical mapping (0x1e50000 - 0x1fa0000)
dtlb_miss_handler c00040c8
itlb_miss_handler c00041a8
Built 1 zonelists.  Total pages: 3953
Kernel command line: root=/dev/ram console=ttyS0

   <Lots more Linux kernel messages...>

Serial: 8250/16550 driver $Revision: 1.90 $ 4 ports, IRQ sharing disabled
serial8250.0: ttyS0 at MMIO 0x90000000 (irq = 2) is a 16450

   <Lots more Linux kernel messages...>

VFS: Mounted root (ext2 filesystem) readonly.
Freeing unused kernel memory: 104k freed
init started:  BusyBox v1.4.1 (2007-03-22 18:53:56 EST) multi-call binary
init started:  BusyBox v1.4.1 (2007-03-22 18:53:56 EST) multi-call binary
Starting pid 22, console /dev/ttyS0: '/etc/init.d/rcS'

Please press Enter to activate this console.

This takes a simulated time of about 37 seconds, and on a modern PC an elapsed time of around 20-25 seconds (the Or1ksim ISS in this minimal configuration runs at 150-200MHz ^[11]).

At this point hitting return will start up a Linux shell, running some basic commands and in this example the BusyBox utilities (see the website for more details).

Please press Enter to activate this console. 
Startingpid 25, console /dev/ttyS0: '/bin/sh'


BusyBox v1.4.1 (2007-03-22 18:53:56 EST) Built-in shell (ash)
Enter 'help' for a list of built-in commands.

# ls /proc
1              2              bus            iomem          self
10             25             cmdline        ioports        slabinfo
11             26             cpuinfo        kcore          stat
12             3              crypto         kmsg           sys
13             4              devices        loadavg        sysrq-trigger
14             5              diskstats      locks          sysvipc
15             6              driver         meminfo        tty
16             7              execdomains    misc           uptime
17             8              filesystems    mounts         version
18             9              fs             net            vmstat
19             buddyinfo      interrupts     partitions     zoneinfo
# busybox mount
rootfs on / type rootfs (rw)
/dev/root on / type ext2 (ro)
proc on /proc type proc (rw)
#

The importance of choosing a suitable value for the global quantum is well illustrated here. Rebuild the model with a global quantum of 100μs—rather longer than the time it takes to transmit one character at 115,200 baud.

#define QUANTUM_US     100

The time taken to boot is marginally faster (19s), but this time the terminal cannot cope with the erratic interrupt behavior.

VFS: Mounted root (ext2 filesystem) readonly.
Freeing unused kernel memory: 104k freed
init started:  BusyBox v1.4.
Please press Ent

The Linux serial driver loses interrupts and the system locks up and will eventually crash with an unhandled interrupt exception.

^[11] This may seem exceptionally fast for an interpreting ISS, but this model is configured with slow RAM with a 20-25 cycle access time and no caches. So 150-200MHz represents only 5-10 MIPS. That's why booting a basic Linux kernel takes 37s of simulated time, rather than the 2-3s that might reasonably be expected!