scripting

This is a trivial REPL-based "scripting thing", designed to run on Z80-based CP/M systems.

The REPL allows arbitrary I/O to ports, and RAM, along with string output and looping-support, but there is little else - for usefulness the currently-selected I/O port is displayed in the REPL prompt.

All-told the interpreter, when compiled as REPL, requires approximately 800 bytes.

Overview

This repository contains a simple REPL-based interpreter which will run upon a CP/M host. Although there is a FORTH-like flavour to the idea of using simple tokens to define actions this is actually not a stack-based language at all.

There are three registers which are used, internally:

• Any time a number is encountered it is moved into a scratch-area.
  • This is notionally known as the accumulator register.
• It is possible to move the accumulator value into the IO-register, U.
  • This register contains the port-number that any I/O read, or write, operation is applied to.
  • Not a great mnemonic, but close to both I&O on the keyboard.
• It is possible to move the accumulator value into the memory-register, M.
  • This register specifies the RAM address of any memory read/write operations.
• Finally you may move the contents of the accumulator into the K-register, which is used for operating loops.

TLDR:

• Numbers go to A-Register.
• Port I/O is controlled by the U-register.
• RAM read/write is controlled by the M-register.
• Loops are controlled by the K-register.

Instructions

The following instructions are available:

• [0-9]
  • Build up a number, which is stored in the accumulator / A-register.
• [ \t\n]
  • Ignored.
• {..}
  • Looping construct, see below for details.
• u
  • Set the I/O-port to be the value of the accumulator.
• U
  • Set the accumulator to be the value of the I/O-port.
• _
  • Print the string wrapped by by "_" characters.
• c
  • Clear the number in the accumulator. (i.e. Set to zero).
• g
  • Perform a CALL instruction to the currently selected RAM address in the M-register.
• h
  • HALT for the number of times specified in the accumulator.
  • This is used for running delay operations.
• i
  • Read a byte of input, from the currently selected I/O port (U-register), and place it in the accumulator.
• k
  • Copy the contents of the accumulator (lower half) into the K-register which is used for loops.
• K
  • Copy the contents of the K-register to the accumulator.
• m
  • Write the contents of the accumulator to the M-register, which is used to specify the address to (r)ead and (w)rite from.
• M
  • Write the contents of the M-register to the accumulator.
• n
  • Write a newline character.
• o
  • Write the contents of the accumulator to the currently selected I/O port (held in the U-register).
• p
  • Print the value of the accumulator, as a four-digit hex number.
• P
  • Print the value of the lower half of the accumulator, as a two-digit hex number.
• r
  • Read the contents of the currently selected RAM address, held in the M-register), and save in the accumulator.
  • Then increment the RAM address held in the M-register (so that repeats will read from incrementing addresses).
• q
  • Quit, if we're in REPL-mode.
• w
  • Write the contents of the accumulator (lower byte only) to the currently selected RAM address held in the M-register.
  • Then increment the RAM address held in the M-register (so that repeats will write to incrementing addresses).
• x
  • Print the character whos ASCII code is stored in the accumulator.

Looping

The special K-register can be used to control how many times a loop will be carried out.

Loops consist of code between { and } pairs. For example the following program will show a countdown:

[00]>10k{Kp}
0009
0008
0007
0006
0005
0004
0003
0002
0001
0000

First of all the value 10 is loaded into the accumulator, then copied into the K-register. The body of the loop is then:

Kp

K copies the contents of the loop-register back to the accumulator, which is then printed by the p command.

Another example would be to dump the first 16 bytes of RAM:

> 0m 16k{rP _ _}
C3 03 EA 00 00 C3 06 DC 00 00 00 00 00 00 00 00

• 0m: Stores the address zero in the M-register
• 16k: Sets the K-register, our loop counter, to 16.
• {: loop-start
  • r: Read a byte into the accumulator from the address in the M-register, incrementing that register in the process.
  • P: Print the contents of the accumulator as a 2-digit HEX number.
  • _ _: Output a space, to split the output nicely.
• }: loop-end

Sample "Programs"

Also note that I broke up the "programs" with whitespace to aid readability. This still works, spaces, TABs, and newlines are skipped over by the interpreter.

Show a greeting:

_Hello, world!_

Store the value 42 in the accumulator, and print it as a four-digit hex number:

42p

To print the value as a byte, not a word use P rather than p:

42P

Store the value "201" (opcode for RET) at address 20000, and CALL it, this will execute RET which will return to our REPL:

20000m 201w 20000mg

Jump to address 0x0000, which will exit back to the CP/M prompt:

0mg

Store the value 42 in the accumulator, then print that as if it were the ASCII code of a character (output "*"):

42x

Configure the I/O port to be port 0x01, read a byte from it to the accumulator, then print that value:

1u i p

Write the byte 32, then the byte 77, to the I/O port 3.

3u 32o 77o

To be more explicit that last example could have been written as:

• 3u32o77o
  • 3 - Write 3 to the accumulator.
  • u - Set the I/O port to be used for (i)nput and (o)utput to be the value in the accumulator, i.e. 3.
  • 32- Write 32 to the accumulator.
  • o - Output the byte in the accumulator (32) to the currently selected I/O port (3)
  • 77- Write 77 to the accumulator.
  • o - Output the byte in the accumulator (77) to the currently selected I/O port (3)

Porting

We use the CP/M BIOS calls for simplicity, if you wished to port this code to a single-board Z80-based system, without CP/M, that would not be hard.

All the system-integration is contained within the file bios.z80 so you could replace the functions appropriately:

• Add your UART initialization to bios_init.
• Update the code to read/write to the serial-console, or whatever, in the other I/O functions.

Inspiration

https://blog.steve.fi/simple_toy_languages.html