We saw in chapters 6 and 12 several control structures but we left out a usual one: the switch also known as select/case. In this chapter we will see how we can implement it in ARM assembler.

It may be suprising, but the ARMv6 architecture does not provide an integer division instruction while it does have a floating point instruction in VFPv2. In this chapter we will see usual ways to workaround this limitation with different techniques that can be used in specific scenarios involving divisions.

If you ever need to use the acts_as_list gem in Rails on a model that uses single table inheritance, here’s the snippet you need to use for the list methods to work if you want the setup done on the base model:

acts_as_list :scope => [:type]

You’ll need to use the array syntax as neither the string nor the symbol versions will work. The symbol one assumes the column ending in _id, while the string one will simply not allow you to evaluate the current object’s type.

It’d be nice to have a lambda syntax in future versions of the gem so you can inject code into the conditions.

We have been using nagios (more specifically Check_MK) recently at work to get some monitoring information on our CentOS instances. Recently we decided to reprovision all of our EC2 instances to apply several security upgrades. Among the packages upgraded, there was the kernel (which I guess was the cause of our subsequent problems).

After updating all our instances, nagios began to complain about the mount options no being the right ones for the root file system, and started sending critical alarms. The file system was ok, it was mounted without problems and everything was woking fine, but for some reason the mount options had changed after the reprovisioning.

Turns out that Check_MK checks the options in place when it does the initial inventory, and if the options change over time, it issues an alarm. If you face this problem, just do a reinventory of your machines and reload the configuration and restart the service, and it should be fine:

cmk -II && cmk -R

In chapter 13 we saw the basic elements of VFPv2, the floating point subarchitecture of ARMv6. In this chapter we will implement a floating point matrix multiply using VFPv2.