Introduction

We examine the ATOM structure in Frits van der Wateren Lisp 1.5. Motorola 8-bit CPU, MC6800.

The code snippet is listed below.

Cell Diagram for PLUS Atom

2021-08-11-PLUS-Memory Layout.png

FDB means initialize double bytes (addresses).

FCC means initialize single bytes.

Memory Layout for PLUS Atom

2021-08-11-PLUS-Memory Layout.png

Comments

Strings Are Tokenized

At the low-levels, this Lisp does not deal with strings.

This Lisp deals only with 4-byte cells, which have been chosen to fit the 8-bit CPU.

In this 8-bit CPU, an address is 16 bits, which is 2 bytes.

CAR is 16 bits (2 bytes).

CDR is 16 bits (2 bytes).

A CONS cell is 32 bits (4 bytes) - one CAR and one CDR.

Magic Marker

Addresses are all mod(4).

Hence, the bottom 2 bits are “free”.

The Magic Marker, called “M”, is set to 000000001 in the address to signify a print-name.

Assembler Directives

The assembler program converts human-readable mnemonics into binary form.

The assembler is line-oriented, i.e. one statement per line.

The assembler assumes that all addresses/offset are byte-oriented.

Addresses/offsets are integers in the range 0-65535.

Labels begin in column 1, all other statements begin after column 1.

The assembler makes 2 passes over the code:

Determine the location of all labels
Convert mnemonics into binary.

*

* here - denotes the current binary offset of the beginning of the line.

‘*’ in column 1 creates a comment (to end of line).

The assembler uses a small set of directives that emit no binary.

EQU

The EQU directive equates a label to a value.

Usually, the value is *, in which case the directive sets the value of the label to the current location of the line in the binary.

Example: OBL28 EQU * means to set the label OBL28 to be the offset of the current line.

Example: *+4 means the offset of here plus 4

FDB

FDB initializes “double” words, i.e. 2 bytes.

FDB uses comma-separated arguments and created 2 bytes for each given argument.

Example: FDB *+4,OBL29 means to initialize 4 continguous bytes. 2 bytes hold here plus 4, the next two bytes are initialized to the integer offset of OBL29.

FCC

FCC initializes single bytes.

Strings of byte-sized characters (ASCII) are surrounded by single quotes.

Example: FCC 'PL' means to initialize 2 bytes, the first to the ASCII value of P (0x50 hex, or 80 decimal) and the second byte is initialized to the ASCII value of L (0x4C hex, or 76 decimal).

Code

Frits van der Wateren Lisp 1.5

Code Snippet For PLUS ATOM

OBL28   EQU   *
   FDB   *+4,OBL29
   FDB   *+12+M,*+4
   FDB   SUBR,*+4
   FDB   PLUS,NIL
   FCC   'PL'
   FDB   *+2
   FCC   'US'
   FDB   NIL
OBL29   EQU   *

PLUS Atom Lisp