Why do we need a preloader and a bootloader in many applications before even the main program

Question

I have read the datasheet of Altera's new HPS. It shows a chart where it shows: Boot ROM -> Preloader -> Bootloader -> OS

I am wondering, why do we need a "pre-loader" and than the "bootloader" and than the OS? I have already read questions concerning bootloaders on this great forum and have some idea of what it is supposed to do. I understand that there are different type of bootloaders and their purpose is not the same in every case.

What I am confused about is this thing that we describe as preloader, why is it there?

Answer 1

Diagram of Loading Process

It doesn't sound like you read very far into the datasheet. From page A-4:

Preloader

The function of the preloader is user-defined. However, typical functions include initializing the SDRAM interface and configuring the HPS I/O pins. Initializing the SDRAM allows the preloader to load the next stage of the boot software (that might not fit in the 60 kilobytes (KB) available in the on-chip RAM). A typical next software stage is the open source boot loader, U-boot. The preloader is allowed to load the next stage boot software from any device available to the HPS. Typical sources include the same flash device that contains the preloader, a different flash device, or a communication interface such as an EMAC.

This is from the datasheet pcj50 linked.