Palo Alto Tiny Basic for RISC-V

This is a port to the RISC-V architecture of Li-Chen Wang's Palo Alto Tiny Basic version 3, (PATB) as published in July 1977 by People's Computer Company.

Build and run

The code shared here is meant to be run inside qemu, but porting it to another system should not bee too difficult. The input/output functions that need to be adapted are located near the end.

It should run on any RV32I implementation. It does not need any extension and it does not make misaligned memory accesses.

Build with

 make

Run with

 make qemu

Press Ctrl+A, X to exit.

ABI and conventions

Warning! This code does not adhere strictly to the standard ABI and calling conventions for RISC-V!

• ra is used as the return address and sp for the stack.
• Registers t0 to t6 are used as temporaries
• There is no hard rule about callee-saved and caller-saved registers
• Function arguments and return values are not always passed via a0

Notable differences with the original source code

The comments and overall program structure have been kept as faithful to the original version as possible, but the differences between the 8080 and RV32 architectures have caused a few deviations described below.

Integer size

Since I am porting PATB to a 32 bits architecture, I have decided to change the integer size from 16 bits to 32 bits. This applies to all arithmetic operations and to the variables' size but not to line numbers, which must still fit in 16 bits. It should not cause any issue with old programs because the original code used to treat any overflow as an error that interrupted the program anyway (a "How?" error).

Each numbered line in program memory is aligned on a 2 byte boundary so that the line number can be read in a single memory access.

Abandoned subroutines

Running on a 32 bits architecture has rendered some helper routines useless: subde, chksgn, chgsgn, chklde, comp, locr were all used to perform 16 bits operations on pairs of 8 bits registers and were not reimplemented. Standard RISC-V instructions acting on a single register are used instead.

The logic behind testch, used everywhere via the macro testc (not shown or explained in the original article), has not been reproduced. Instead, ignblk returns the first non-blank character ready to be compared.

No mixed uses of calls and tails

In the Intel world, the call instruction is responsible for pushing the return address on the stack before jumping to the function to execute. This function can then be used either as a plain function (called, return), or as a tail call (jump into it, return).

In RISC-V, since it is up to the called function to save the value of the ra register on the stack, this approach is not practical. Such a block of code can be used either as a plain function (saving ra in its preamble, restoring it before returning), or as a tail-call or fall-through (restoring ra that was saved from somewhere else), but not both.

This has forced a few little changes here and there, but more prominently in parn, that had to become a real function, and in size, that cannot be used as one.

Random

The random generator I used is converted from an x86 assembly code snippet shared on the VCFed forums: https://forum.vcfed.org/index.php?threads/palo-alto-tiny-basic-download.40493/post-492806

The seed is not randomized and the generator will always output the same sequence upon restart.

Links

A copy of the original article from the PPC's Reference Book of Personal and Home computing can be found on the Internet Archive: https://archive.org/details/Palo_Alto_Tiny_BASIC_Version_3_Li-Chen_Wang_1977/mode/1up

The Tasty Basic project on Github has a some nice additional example programs to play with.