Hello – Tiny ROM

This variant of the Hello World project demonstrates how to minimise code size for the ZPU core.

In the Makefile we specify the minimal minctr0.s (as opposed to the full version). This omits the emulation table for optional instructions, so if we use this startup code we must make sure the code doesn’t contain any instructions not implemented by the CPU. This is done by enabling optional instructions when instantiating the ZPU core. In this instance we use:

	IMPL_STOREBH => false,
	IMPL_LOADBH => false,
	IMPL_CALL => true,
	IMPL_SHIFT => true,
	IMPL_XOR => true,
	REMAP_STACK => false,
	EXECUTE_RAM => false,

(Note: we don’t enable hardware LOADB/H or STOREB/H because they currently only work when addressing external RAM, and not the internal Boot ROM/Stack RAM.) However, there are still instructions not implemented in hardware and we must make sure GCC doesn’t emit them, by way of compiler flags.

ZPUOPTS= -mno-poppcrel -mno-pushspadd -mno-callpcrel -mno-byteop \
	-mno-shortop -mno-neg

There are a few pitfalls to be aware of:

  • -mno-neg is broken in the binary GCC toolchains available on the net. If you build the toolchain from source using the current repository this won’t be a problem.
  • Load / Store B/H are problematic for data in the Boot ROM, so we ask GCC to avoid emitting those instructions.
  • Division and modulo are a problem. They’re expensive to implement in hardware, and while it’s possible to link in software division from libgcc the implementation there uses other instructions that we’re trying to avoid – so for now, you can’t use division or modulo in code built with mincrt0.s

We still use the standalone_simple.ld linkscript.

The HelloTinyROM code compiles to a mere 252 bytes (as opposed to 1772 bytes for the simple Hello World example), and the DE1 example project, with optional instructions enabled in the ZPU, takes up 998 logic elements.

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