AcornCmosBasic: A Shell repository from BigEd

Introduction
============

This repository contains the original source code for Acorn Cmos Basic.

The structure of the repository is as follows:

    original_sources.zip
              - a copy of the original Cmos Basic source files,
                with the original <cr> line endings.

    dfs/      - disk images in DFS (ssd and dsd) formats;
                these are generated by the make_disk_images.sh script
                which uses SWEH's Perl mmb_utils.

    src/      - the Cmos Basic source files (with unix line endings)

    tools/    - binaries for MASM (both DFS and non-DFS versions)

Assembling the OS Source Code
=============================

An "ordinary" 6502 Co Processor is needed to assemble the Cmos Basic
sources.

The source code is in Acorn MASM format, and a copy of the "Normal"
version of MASM is included in the disk images.

Using DFS:
==========

(or MMFS)

The disk organization on DFS involves two disk images:
- drive 0: Tools (MASM) and Working files
- drive 2: Cmos Basic sources

In the dfs/ directory are following versions:

Two seperate single sided disk images:
    ssd/AcornCmosBasic_disk0.ssd
    ssd/AcornCmosBasic_disk2.ssd

One double-sided disk image:
    dsd/AcornCmosBasic_disk02.dsd

Transfer one of these disk image sets onto your physical hardware.

To assemble the sources, you just need to boot disk 0 (it contains an
appropriate !BOOT file)

The Basic binaries are generated on DRIVE 0 (two files):

$.CBasObj
    (this has a MD5SUM of 4b484a2011596dca83167a1feffbd6d7)

Notes
=====

So, exactly which version of BBC Basic is this?

The copyright string is (C) 1984 and the Version is 04. This matches
Basic 4 from the Master MOS 3.20 Mega ROM. However, the binary is
different.

If you compare the two binaries, the difference turns out to be minor:
a one-line addition to CBAS04 (line 469):

SMULB ASL IACCL
 ROL IACCM
 BCS NOTGO   ;; <<<<<<<<<<< Added after Basic 4 released
 LDA WORK+8
 ORA WORK+9
 BNE SMULA
 ASSERT :MSB: SMULA = :MSB: .
 STY IACCL
 STX IACCM
 RTS
NOTGO BRK
 = &0A
 = "Bad ",TDIM
 BRK

This looks like the DIM fix talked about here:
http://beebwiki.mdfs.net/DIM_Basic432

If you comment this line out, then the generated binary matches Basic
4. So this effectively is the source to Basic 4.

Acknowledgements:
=================

Many thanks to Paul Fellows (ex Acornsoft) for discovering these
long-lost sources and making them available to the Acorn community.
BigEd/AcornCmosBasic
