assembly RISCV RV32IC lz4 decoder

lz4 decoder for RISCV RV32I CPUs in assembly language
pointer to packed data in a0 on entry
pointer to depack destination in s0 on entry
used: a1,a2,a3,a4,a5,a6 for compliance with C-extension
size 116 Bytes for RV32IC
168 Bytes for RV32I

Code assumes a 2 byte header (little endian size) and then
the raw compressed BLOCK in lz4 format (up to 64 kB).
Compressed for example with lz4 -12 -B4 and header + tail cropped.
(See Makefile).

Now it is even listed on the official LZ4 page. ;-)
Another bare metal example is here: mandelbrot in under 1kb.

application in bare metal assembler

packedearth.asm is a bare metal assembly example for the Longan Nano.
It displays an lz4 compressed image (160x80) of the earth map in 565 format (25.6 KB).
Total size: 4624 Bytes.

The decoder and example code assembles well with the wonderful bronzebeard.

usage

make
make flash

including in own code

You can of course either assemble the source as you need or even simply include the binary 'blob' lz4depack.bin at any arbitrary position (aligned to 16bit with C extention).

The routine uses only relative short jumps and is therefore fully relocatable even in binary form.
It simply requires a0 and a1 to be set before call. a1,a2,a3,a4,a5,a6,t0 are used and NOT saved.

Assemble with GCC instead of Bronzebeard

As kindly suggested by brucehoult you can make the register defines gcc/gas compatible:

perl -pe 's/^\s*([a-z_]+)\s*=\s*([a-zA-Z0-9]+)(\s*#.*)?$/#define $1 $2/' lz4depack.asm >lz4depack.S

Then assemble, using the C preprocessor:

riscv64-unknown-elf-gcc -march=rv32ic -mabi=ilp32 lz4depack.S -c

And check the code using riscv64-unknown-elf-size or riscv64-unknown-elf-objdump

Licensed under the 3-Clause BSD License Copyright 2021, Martin Wendt