Overview

This page describes the layout of the different memory regions, since the use of a bootloader makes this more complicated than the usual PIC32 program.

As background, read the chapter on Memory Organization from the PIC32 Family Reference Manual. It's important to understand the difference between virtual and physical memory addresses, the different regions (Boot vs Program Flash vs RAM) and the difference between kseg0 and kseg1. We don't use kuseg so you won't see virtual addresses less than 0x8000000.

It's also worth reading the PIC32 Bootloader example since our memory layout is based on the one described there.

This page assumes a MX795F512L, since that's what is used on the Ethernet Starter Kit. If you're using another chip you'll need to adjust the linker scripts & the boot loader constants according to the amount of program flash / RAM provided on the chip.

Hardware constraints that influence the design include:

• Flash must be erased before programming (write), after an erase the flash contains all 1s.
• Flash can only be erased a page at a time (4kB chunks)
• Flash has a limited (rated for 1000) number of write cycles
• Flash is written either 4 bytes at a time, or 512B at a time.
• While writing flash, the program execution stalls.

Memory Organization

Boot / Program Flash

The MX795F512L has the following (physical) memory regions:

• 128kB of RAM [0x00000000 - 0x0001ffff]
• 12kB of Boot flash 0x1fc00000 - 0x1fc02fef]. The upper 4k of the boot flash is used for the debug executive (in circuit debugging and the Device Configuration Registers.
• 512kB of Program Flash [0x1d000000 - 0x1d07ffff]
• SFRs, including peripherals, which aren't important for this topic.

Since our bootloader (16k) is too big to fit in the boot flash we place it in the lower part of the program flash. However the boot flash is still used for the initial code (.reset) and the interrupt vector table (IVT). This minimises the amount of program flash used by the bootloader.

Since the early models don't have an EEPROM, we also store the device's UID in program flash. We could have embedded the UID in the bootloader but this means you need a custom bootloader image for each device.

The downside of storing the UID in program flash is that we consume an entire page (4kB) for a 6 byte UID.

The main application comes with it's own IVT that we need to reserve room for. We put this in the first 4k of the application's address range, and then the program code follows it.

Putting this all together, the Program Flash layout is:

Finally, we use the Program Flash Write Protect (PWP, see the data sheet for the chip) feature to avoid a bug accidentally overwriting the bootloader. PWP is at the page-level so we write protect the first 6 pages (24k).

RAM

During the firmware update procedure, the application needs a way to signal to the initial code that it should enter the bootloader rather than boot the main application. We call this the boot_option.

We could use flash for this, but we'd have to dedicate another entire page of flash (4kB) since that's the smallest granularity of a flash-erase and it would persist across power-cycles, which may not be what we want.

Instead, we store the Boot Options in the upper range of the RAM address space. To enter the bootloader, the application writes the desired boot option to the boot_option memory location and then triggers a software reset.

We reserved 16B in case we need to transfer any other data between the bootloader and the application.

Linker Scripts

The linker is responsible for taking the object files (.o) and combining them to form a single executable. The linker configuration is specified through the use of a linker script, which controls the mapping of data to memory regions.

The MPLAB® Assembler, Linker and Utilities for PIC32 MCUs User’s Guide covers the different types of memory sections and the use of linker scripts.

We have a custom linker script for both the bootloader and the application. These can be found in the linker directory.

References

• Section 3, Memory Organization
• PIC32MX5XX/6XX/7XX Data sheet
• PIC32 Bootloader example
• MPLAB® Assembler, Linker and Utilities for PIC32 MCUs User’s Guide MPLAB® Assembler, Linker and Utilities for PIC32 MCUs User’s Guide