The assembler is a two-pass assembler. The input file consists of 5 "commands" in the following format :
.soperand = #sym sym @sym
symbol value
...
.a address
.b sym sym ...
.c
:label opcode operand
...
.e
.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
@ jump relative
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 fields are allowed. 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.
.sThe output S-format file that is ready to be used to write into a EEPROM is :
print 1000
stop 1001
half 100
temp 101
temp2 102
base 2
.a 0
.c
:begin
lds #300
lds temp
lda #30
lda temp
psh
sta half
jmp begin
jmp @begin
jpz next
jpc @begin
jsr next
and #20
and temp2
pop
add #40
add data
rts
rol
ror
:next
jsr print
:data
.b 1 2 3
.e
S113000004012C0C0065141E1C0065203C00644C8B
S1130010000048EC5C002A68E77C002A84148C0009
S113002066A0C428CC002DD0E0F07C03E8010203D4
S9030000FC
The main objective of this implementation of a4 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
20 July 2004
Prabhas Chongstitvatana