Posted on

Virtual ROM on small FPGAs

Readonly data – outsourced

When running display output applications on small FPGAs where printing of strings is required, using the internal block RAM for character sets would be a waste of resources, or even: not sufficient. The 64kB character map for a normal and inverted font using both RGB and BGR subpixel smoothed renderings (more below) would eat up more than available on the Spartan3-250k device, for example.
The Spartan3 on my good old Papilio has a SPI flash attached with less than 50% actual usage for the FPGA bit stream. Tadaa. Plenty of space for character bitmaps. So we can just store the bitmap in an unused sector on the SPI and blit-copy from SPI flash to the LCD display directly.
But what if there’s more read-only data, like second stage program code or coprocessor microcode that is loaded on the fly by an applet? If we don’t need it for the boot process, it should not live in the block ram permanently, but still execute from block RAM. Screams for a cache, doesn’t it?
So the simple solution actually is, to create a small controller entity ’scache‘ inside the system specific peripheral of the SoC. This simply watches access to certain addresses and generates an exception once this address is hit. The exception vector then jumps into a handler routine which does all the SPI flash loading into the physical cache memory area. Then, for the next LOAD instruction, the virtual address internally translates to the physical cache address.
This requires very simple logic, however runs through some program code and needs a bit of time to load from the SPI flash.
Turns out this is barely noticeable for the LCD display.

Under the hood: Linker scripting

Ok, so there is plenty more data from the .rodata section. We could implement some kind of overlay loader and a kind of file system, but why make it complicated, when our data is somewhat static. We just relocate the cached external data using a linker script. Say, our program memory ranges from 0x0000 to 0x2000, the cache is allocated after that. Then we define a memory specification in the linker script as shown below:

        l1ram(rwx): ORIGIN = 0x0000, LENGTH = 0x2000
        l1cache(rwx): ORIGIN = 0x2000, LENGTH = 0x2000
        xdata(r): ORIGIN = 0x10000, LENGTH = 0x8000

For the .rodata section, we can simply use a line like

.rodata         : { *(.rodata .rodata.* .gnu.linkonce.r.*) } > xdata

to allocate the read-only areas into the virtual SPI ROM starting from 0x10000. Eventually, compiling the whole story will result in an ELF file which we only have to separate into the boot ROM area and the second stage binaries that live in the SPI flash.

But we may want to have read-only data that should be present at all times, like library code that is frequently used. For this purpose, we can just define our own segments and use the __attribute__ decorator for the data that we want in BRAM:

__attribute__((section(".l1.rodata"))) char g_fifobuf[32];

I will not dive into the details of linker scripting, but likewise you can also allocate entire library or object files in specific memory segments.

This simple technique allows you to pack quite a bit of code into small FPGA SoCs.

The LCD example

LCD screen
LCD subpixel smoothing example

Here you can see an example of the character table in action. I have mentioned the RGB/BGR subpixel smoothing above which lead to a bit higher ROM usage. The used LCDs have quite a bit of functionality, like setting the orientation of the screen display. You could just not care about when designing a character table and use a simple black/white scheme. However, when trying to achieve a 4×8 pixel character set, the font will look quite unreadable and you’re better off with the subpixel smoothing. This again is sensitive to the pixel order, so for top and bottom display orientation you will already need the font rendered in two variants, plus in color. Now, as we have the space, it is just much simpler to drop the entire bit map into the flash instead of figuring out compression techniques.

Program code

Likewise, not too frequently used program code can be dropped into external SPI flash. This comes in handy when a program becomes bigger during development and the space usage can not be determined a priori. The ZPUng is able to handle a specific exception signal that is risen by the SCACHE controller. The cache handler microcode routine then loads code from SPI flash and resumes execution while translating the virtual PC address into a physical address inside the cache area, where instructions are effectively fetched from. This is the trick used to run the netpp communication library for IoT applications  on FPGAs with limited resources, like the Papilio One.