Most of banks use linux boxes for downstream systems (reporting and so on) but a mainframe is the centrepiece of it all, executing end of day batch jobs (this can be anything from the feeds to the clearing house, or end of day reconciliation) and processing trades.

You will be surprised, but it is due to the fact that banks were some of the first people to start using computing decades ago, and some of the programs I have seen in use at banks were older than me. Moving everything to a Intel based platform removes that backwards compatability, but z/OS is made with this in mind.

> Most of banks use linux boxes for downstream systems (reporting and so on)

I don't know - for regular client-server computing, I've seen a mix at the 6 banks I've worked at - some Linux, some Windows Server, some Solaris. Most banks have a mix of all three, but have one that they "major" in.

Also, mainframes tend to be more of a Retail banking thing, for the core banking platform.Some Investment Banks use mainframes for back office stuff, but not many (in my experience).

Where I am working now, I have seen plaques in recognition for using a particular mainframe program from VISA for 20+ years. Also, the recently-retired core-banking platform from an acquired retail bank ran on mainframes, but was written in assembler.

"process huge amount of data"

What kinds of workloads though? I can see that "supercomputers" might be appropriate for modelling, analysis and reporting but that's a very different kind of scenario to highly available high throughput transaction processing that mainframes have traditionally excelled at.