How to use the assembler A3

command line

	a3 inputfile 

will generate two files :  file.lst  listing, file.s  S-format object

The assembler is a two passes  assembler.   The input file consists of 5 "commands" in the following format :

.s                        
symbol value
 ...
.a address                
.b sym sym ...           
.c                        
:label opcode operand
 ...
.e                       

operand = #sym sym +sym
sym = numeric symbol label
numeric begins with 0..9
symbol  found in the symbol table

.s  start the symbol defining section.  A symbol is associated with its numerical value.  When the assembler encounters a symbol in the input file it will substitute that symbol with its value.  
.a  set the address.  It is used to define a location.
.b  define constant in the memory.  It is useful when building a table of value or to assemble a sequence of values even inside code segment.
.c  code segment.  
.e the last line of program

The addressing modes are specified with prefix :  
  #  immediate
  +  index
without any prefix the symbolic operand is considered to be direct address.

.s .a .b .c  can occur in any sequence.  Input scanning is case insensitive.  Internally all symbols are converted into uppercase and stored in the symbol table.  No comment field allows.  You can implement one easily using another program to "filter" out all the comments.  For example, a  comment can be defined as anything after  ;; or // until the end of line.

Example 
.s
print	1000
stop	1001
half 	100
temp 	101
temp2 	102
base	2
.a  0
.c
	lda #15
	sta half
	cla
	add #1
	com
	and half
	ora #128
	sta temp
	xor temp
	lda half
	cla
	jz next
	jmp next
:next 	ldx #257
	inx
	dex
	stx temp2
	adc +3
	ora +base
	jsr  print
	jsr  stop
:data
.b 1 2 3
.e

The output S-format file that is ready to be used to write into a EEPROM is :

S11300001D0F220064180101080E006411802200F3
S1130010651600651E0064183A001E46001E550150
S1130020015C605A0066070313024A03E84A03E9C5
S1060030010203C3
S9030000FC

The main objective of this implementation of a3 assembler is to make it as simple as possible.  The source code is only 200 lines long.  It is a two passes assembler.  The first pass collects all the symbol and counts the address.   The second pass does the actual generation of machine code.   As this is the first release, it is not yet robust.   Please report any bug found to  prabhas@chula.ac.th

2 October 1999
Prabhas Chongstitvatana
