FLEX ASM09 Syntax Reference

Target System: FLEX Operating System (6809) CPU Support: Motorola 6809, 6800, 6801 Source: ASM09 6809 Optimizing Assembler Version 3.2 Purpose: Complete syntax reference for xasm++ FLEX compatibility

Overview

FLEX ASM09 is the assembler for the FLEX Disk Operating System (FLEX DOS) developed by Technical Systems Consultants (TSC). It's a sophisticated 2-pass assembler with macro support, conditional assembly, and listing control.

Key Features

2-pass assembly with symbol resolution
Macro preprocessor with parameters
Conditional assembly directives
Listing control with pagination and titles
Library support for modular development
Expression evaluation with full operator set
Backward compatible with 6800/6801 code

Differences from EDTASM

Feature	FLEX ASM	EDTASM
Macros	✅ Full support	❌ No macros
Conditional Assembly	✅ IF/ENDIF	❌ Limited
Listing Control	✅ NAM, TTL, PAG, SPC	❌ Basic
Libraries	✅ LIB directive	❌ No libraries
Comments	`*` or `+`	`;` or `*`

Source Line Format

FLEX ASM uses a multi-field format:

[LABEL] [OPERATION] [OPERAND] [;COMMENT]

Label Field

Position: Column 1
Format: 1-8 alphanumeric characters, starting with letter
Terminator: Space or tab (no colon needed)
Special: * or + in column 1 indicates comment line

START    LDA    #$42      ; Label in column 1
LOOP     INX              ; Another label
         STA    BUFFER,X  ; No label

Operation Field

Mnemonics, directives, or macro calls:

Mnemonics: 6809 instruction set
Directives: ORG, EQU, FCB, etc.
Macros: User-defined macro names

Operand Field

Expressions, addresses, or directive parameters.

Comment Field

Preceded by semicolon ;
Continues to end of line
Also: Entire line comments start with * or + in column 1

Directives

Program Structure

ORG - Set Origin

ORG    $8000        ; Set assembly address

Sets the current assembly address.

END - End of Source

END    START        ; End with entry point
END                 ; End without entry point

Marks end of source file. Optional parameter specifies program start address.

SETDP - Set Direct Page

SETDP  $00          ; Direct page at $0000
SETDP  $10          ; Direct page at $1000

Informs assembler of direct page register value for optimization.

Symbol Definition

EQU - Equate (Constant)

SCREEN  EQU   $0400      ; Define constant
ROWS    EQU   24         ; Numeric constant

Defines a constant symbol (cannot be redefined).

SET - Set Variable

COUNTER SET   0          ; Define variable
COUNTER SET   COUNTER+1  ; Can be redefined

Defines a variable symbol (can be reassigned).

Data Definition

FCB - Form Constant Byte

        FCB   $01,$02,$03     ; Bytes
        FCB   'A','B','C'     ; Characters
        FCB   <ADDR           ; Low byte
        FCB   >ADDR           ; High byte

Defines byte-sized data.

FDB - Form Double Byte (Word)

        FDB   $1234           ; 16-bit word
        FDB   ADDRESS         ; Address value

Defines 16-bit word data (big-endian).

FCC - Form Constant Character

        FCC   /Hello World/   ; Slash delimiters
        FCC   'String'        ; Quote delimiters
        FCC   "String"        ; Double quote delimiters

Defines character string data. Delimiter is first non-space character after FCC.

RMB - Reserve Memory Bytes

BUFFER  RMB   256           ; Reserve 256 bytes
WORK    RMB   16            ; Reserve 16 bytes

Reserves memory space without initialization.

Listing Control

NAM or TTL - Name/Title

        NAM   Program Title
        TTL   Module Name

Sets program title for listing header.

STTL - Subtitle

        STTL  Section: Main Loop

Sets subtitle for current page.

PAG - Page Break

        PAG                   ; Force new page

Forces new page in listing.

SPC - Space Lines

        SPC   2               ; Insert 2 blank lines
        SPC   5               ; Insert 5 blank lines

Inserts blank lines in listing.

Macro Directives

MACRO - Begin Macro Definition

MOVB    MACRO   SRC,DEST     ; Define macro with parameters
        LDA     SRC
        STA     DEST
        ENDM

Begins macro definition. Parameters are positional.

ENDM - End Macro

        ENDM                  ; End macro definition

Terminates macro definition.

Macro Invocation

        MOVB    $80,$90      ; Call macro

Invokes macro with actual parameters.

Conditional Assembly

IFC - If Condition

        IFC     DEBUG        ; If DEBUG defined
        JSR     TRACE
        ENDC

Conditional assembly based on symbol definition.

ENDC - End Conditional

        ENDC                 ; End conditional block

Terminates conditional assembly block.

Advanced Directives

LIB - Include Library

        LIB     STDLIB       ; Include library file

Includes external source file during assembly.

REG - Register List

        REG     A,B,X,Y      ; Define register list for PSHS/PULS

Defines register lists for stack operations.

OPT - Assembler Options

        OPT     C,L,P        ; Set options

Controls assembler behavior:

C - Enable cycle counting
L - Generate listing
P - Enable pagination

ERR - Generate Error

        ERR     Invalid configuration

Forces assembler error with custom message.

RPT - Repeat Block

        RPT     10           ; Repeat 10 times
        NOP
        ENDR

Repeats following code block N times.

Expressions

Operators

FLEX ASM supports standard operators with C-like precedence:

Operator	Description	Precedence
`+`	Addition	Low
`-`	Subtraction	Low
`*`	Multiplication	Medium
`/`	Division	Medium
`%`	Modulo	Medium
`&`	Bitwise AND	Low
`\|`	Bitwise OR	Low
`^`	Bitwise XOR	Low
`<<`	Shift left	Medium
`>>`	Shift right	Medium
`<`	Low byte	High
`>`	High byte	High

Expression Examples

        LDA     #$FF&$0F      ; Bitwise AND
        LDX     #TABLE+16     ; Addition
        LDA     #>ADDRESS     ; High byte
        LDB     #<ADDRESS     ; Low byte

Number Formats

FLEX ASM supports multiple number formats:

Format	Prefix/Suffix	Example
Decimal	None	`123`
Hexadecimal	`$` prefix	`$1A2F`
Binary	`%` prefix	`%10110101`
Octal	`@` prefix	`@377`
Character	Single quotes	`'A'`

Addressing Modes (6809)

FLEX ASM uses standard Motorola syntax:

; Inherent
        NOP

; Immediate
        LDA     #$42
        LDX     #$1234

; Direct (Zero Page)
        LDA     $80
        LDA     <ADDRESS      ; Force direct

; Extended (Absolute)
        LDA     $1234
        LDA     >ADDRESS      ; Force extended

; Indexed
        LDA     ,X            ; No offset
        LDA     5,X           ; 5-bit offset
        LDA     $80,X         ; 8-bit offset
        LDA     $1234,X       ; 16-bit offset
        LDA     A,X           ; Accumulator offset
        LDA     D,X           ; D accumulator offset
        LDA     [,X]          ; Indirect
        LDA     [,X++]        ; Auto-increment

; Relative (Branches)
        BNE     LOOP
        LBRA    FARAWAY       ; Long branch

Comment Styles

FLEX ASM supports multiple comment styles:

* Full line comment (column 1)
+ Full line comment with page break control

        LDA     #$42      ; End of line comment

; Standalone comment line (any column)

Comment Types:

* in column 1 - Regular comment, page break if <14 lines remain
+ in column 1 - Forces new page before comment
; - End-of-line comment or standalone comment

Macro Features

Parameter Substitution

MOVE    MACRO   SRC,DEST
        LDA     SRC          ; Parameter 1
        STA     DEST         ; Parameter 2
        ENDM

        MOVE    $80,$90      ; Invocation

Local Labels in Macros

WAIT    MACRO   COUNT
.LOOP   DEC     COUNT
        BNE     .LOOP
        ENDM

Local labels (starting with .) are unique per macro invocation.

Example Programs

Simple Program

        NAM     Hello World
        TTL     FLEX ASM Example

SCREEN  EQU     $0400

        ORG     $8000

START   LDX     #SCREEN
        LDY     #MESSAGE
LOOP    LDA     ,Y+
        BEQ     DONE
        STA     ,X+
        BRA     LOOP

DONE    RTS

MESSAGE FCC     /HELLO WORLD/
        FCB     0

        END     START

With Macros

        NAM     Macro Example

; Define macro
PUSH2   MACRO   R1,R2
        PSHS    R1,R2
        ENDM

POP2    MACRO   R1,R2
        PULS    R1,R2
        ENDM

        ORG     $8000

START   LDX     #$1234
        LDY     #$5678

        PUSH2   X,Y          ; Save registers
        JSR     SUBRTN
        POP2    X,Y          ; Restore registers

        RTS

SUBRTN  ; Subroutine code
        RTS

        END     START

Compatibility Notes

FLEX vs EDTASM

Compatible:

Basic directives (ORG, EQU, FCB, FDB, FCC, RMB, END)
6809 instruction mnemonics
Expression syntax

FLEX Additions:

Macro support (MACRO/ENDM)
Conditional assembly (IFC/ENDC)
Library includes (LIB)
Advanced listing control
Repeat blocks (RPT/ENDR)

Migration:

EDTASM programs generally work in FLEX ASM
FLEX programs may use features not in EDTASM

References

Original Manual: ASM09 6809 Optimizing Assembler Version 3.2
Publisher: Southwest Technical Products Corporation
Availability: docs/reference/flex/ASM09-6809-assembler.pdf

Implementation Notes

For xasm++ developers implementing FLEX ASM support:

Code Reuse Opportunities

Extend MotorolaBaseSyntax (~60% code reuse)
- Basic directive handling (ORG, EQU, FCB, etc.)
- Expression evaluation
- Addressing mode parsing
New Components Needed
- Macro preprocessor
- Conditional assembly handler
- Listing control system
- Library inclusion mechanism

Estimated Implementation

Phase 1: Basic directives (10-15 hours)
Phase 2: Macro processor (15-20 hours)
Phase 3: Conditional assembly (8-10 hours)
Phase 4: Advanced directives (10-15 hours)
Total: 45-60 hours

Testing Strategy

Unit tests for each directive
Macro expansion tests
Conditional assembly tests
Integration with existing 6809 CPU
Real FLEX source code compatibility

Overview​

Key Features​

Differences from EDTASM​

Source Line Format​

Label Field​

Operation Field​

Operand Field​

Comment Field​

Directives​

Program Structure​

ORG - Set Origin​

END - End of Source​

SETDP - Set Direct Page​

Symbol Definition​

EQU - Equate (Constant)​

SET - Set Variable​

Data Definition​

FCB - Form Constant Byte​

FDB - Form Double Byte (Word)​

FCC - Form Constant Character​

RMB - Reserve Memory Bytes​

Listing Control​

NAM or TTL - Name/Title​

STTL - Subtitle​

PAG - Page Break​

SPC - Space Lines​

Macro Directives​

MACRO - Begin Macro Definition​

ENDM - End Macro​

Macro Invocation​

Conditional Assembly​

IFC - If Condition​

ENDC - End Conditional​

Advanced Directives​

LIB - Include Library​

REG - Register List​

OPT - Assembler Options​

ERR - Generate Error​

RPT - Repeat Block​

Expressions​

Operators​

Expression Examples​

Number Formats​

Addressing Modes (6809)​

Comment Styles​

Macro Features​

Parameter Substitution​

Local Labels in Macros​

Example Programs​

Simple Program​

With Macros​

Compatibility Notes​

FLEX vs EDTASM​

References​

Implementation Notes​

Code Reuse Opportunities​

Estimated Implementation​

Testing Strategy​