This utility (GTAsm) is intended for small 6809 machine language utilities that enhance programs written mostly in Dragon Basic, for example a routine to scroll the graphics screen. The assembly source code is written in Basic comment statements.

Download GTAsm Inline Basic Assembler (as a .cas file for Dragon 32)

With any small micro with limited RAM there's a big problem: the source code, executable program, and the assembler program itself all have to occupy memory at the same time. As well as the limitation on total space, the executable and the assembler might want to accupy the same address space at the same time.

GTAsm deals with this problem by loading itself into the graphics pages by default; the assembled code can then freely use the high memory area. Obviously this means the assembler will be corrupted and have to be reloaded every time graphics are drawn. To avoid this, reserve extra pages with PCLEAR and load the assembler in these, or load it into the reserved high memory area. The assembler's workspace (for calculating labels etc.) is limited by the end of the graphics pages, or the top of RAM (&H8000) as appropriate.

A typical starter program might look like this:

 10 PCLEAR7:CLEAR200,&H6FFF
500 'ORG$7000:CONST TXBASE=$0400,TXEND=$0600,BLANK=$F6:ON
510 'DEFUSR1:@CLRSCR LDX#TXBASE:LDA#BLANK
520 '@LOOP STA,X+:CMPX#TXEND:BLO LOOP
530 'RTS
540 'END

The PCLEAR statement clears an extra 3 graphics pages, or 4.5K of RAM. This should be enough for short routines, and leaves the first 4 pages untouched so hi-res modes (PCLEAR3,1 etc.) work as normal. The CLEAR statement sets the last byte of Basic RAM, so our machine code program can start at &H7000.

Run the Basic program. Now load GTAsm and execute it with:

CLOADM
EXEC

That's if we're happy for the assembler to sit in graphics pages 1 to 3 and don't want to bother with offsets. The assembler can be loaded anywhere in RAM as long as it starts at a page boundary; the entry point is &H0900 bytes after the start of the code. To load the assembler with an offset to start at graphics page 5:

CLOADM "ASMB0600",&H1800
EXEC &H2700

A trace of addresses and assembled machine code should be listed to screen. If there are no errors, run the assembled routine with:

EXEC &H7000

Or, since we used the DEFUSR1 directive:

A=USR1(0)

EXEC remembers its last address, so as long as we don't use it (or DEFUSR0) for something else we can type plain EXEC to re-run the assembler.

GTAsm expects assembly language mnemonics and directives to be written in BASIC comments beginning with the single quote symbol. Use upper-case only, except for string constants. Multiple instructions on one line are separated by colons. Editing is done as per usual for lines of Basic.

All standard 6809 assembler mnemonics are available, with one exception:

TFR (use TRF instead - this is a simple error)

A useful selection of directives is available, though not all of the standard ones. There is no EQU - use CONST instead.

ORG <addr>                    set the program origin              ORG $7C00
ON                            switch on trace output              ON
OFF                           switch off trace output             OFF
DEFUSR [0..9]                 define Basic USR entry point        DEFUSR1                 
CONST <label>=<word>          comma-separated list of constants   CONST START=100,FINISH=250
VAR <byte>,<labellist>        reserve comma-separated list        VAR 1,TEXTPOSX,TEXTPOSY 
                              of variables of this size           VAR 2,MASKADR,BITMAPADR
FCB <bytelist>                comma-separated list of bytes       FCB $80,$40,$20,$10,"XVID",0
FCC <bytelist>                .. or strings
FDB <wordlist>                comma-separated list of words       FDB $BD52,%7C00
RMB <word>                    reserve bytes of memory             RMB 1000    
SETDP <byte>                  assume DP contains this value       SETDP $7C                          
END                           end of assembly language            END

The assembler is very forgiving about using spaces. Just as in Basic, if a statement can be parsed left to right without ambiguity then spaces can be omitted to save memory.

Hexadecimal values should begin with the '$' character, rather than Basic's “&H” symbol. Strings should be enclosed in double quotes.

You can start a statement with an '@' symbol followed immediately by a label, a space, and an instruction or directive. Labels are alphanumeric (1st character must be a letter), and can be quite long; 16 characters is a useful maximum, bearing in mind memory is limited.

A comment can be added after any valid statement without preceding it with any special character.

When assembly fails there are a number of useful error messages. Beware that some errors can be missed; sometimes you can write garbage amd have it silently interpreted as a comment.

Keep lines to a reasonable length to avoid a bug in Basic. Entering a line of the maximum length then trying to edit it can cause a system crash. This appears to be due to the single-quote being stored as a two-byte token.

Compatability: GTAsm is for the Dragon 32 only. If it works on any other system then that's a happy accident.

https://6809.uk/cas/GTAsm.cas

5 PCLEAR8:CLEAR200,&H6FFF
10 PMODE4,1:PCLS:SCREEN1,1
20 LINE(31,0)-(31,191),PSET
22 LINE(26,0)-(26,191),PSET
30 C2$="BM3,67U0":C3$="BM3,74FGHE":C4$="BM3,81RFD2GL2HU2E":C5$="BM3,88RF2D2G2L2H2U2E2"
40 REMDRAWC2$+C3$+C4$+C5$+"BM3,97RFD2GL2HU2E"+"BM3,106FGHE"+"BM3,115U0"
50 A$="D2R1D2FD2BL4U2EU2BD6D3LNDNU2LNUDLNULDLNUL2BR16L2NULULNULNU2LU3LNUD2BR4UENUFD2BL14U2ENUFDBR5BU4NU5LNU3R2NU3BLBU2C0NU2NFNGBD8C1BRF2L2NUHGNUL2E2BRU2
60 REM DRAW"BM156,100XA$;
70 REM DRAW"BM156,50A2XA$;
90 DRAW"A2BM8,31XA$;A0BM8,0XA$;
200 B$="BR2FD3GHU3EBR2
220 DRAW"BM0,33XB$;XB$;XB$;XB$;XB$;XB$;
230 R2$="RNEFD2NFGL2NGHU2NHER":R5$="C0D2NR2NL2ND2C1NUNRNDNL":R2$=R2$+R5$
235 R3$="RFNED2GNFL2HNGU2ENHR"+R5$
240 R4$="NURFDNRDGLNDLHUNLUER"+R5$
245 DRAW"BM3,65XR2$;BM3,73XR3$;BM3,81XR4$;BM3,89XR2$;BM3,97XR3$;BM3,105XR4$;
250 REM LINE(147,99)-(165,116),PSET,B
300 X=&H7E80:FORJ=0TO31
310 FORK=0TO2:POKEX,PEEK(&H600+J*32+K):X=X+1:NEXT:NEXT
320 X=&H7E00:FORJ=0TO63:POKEX+J,PEEK(&HE00+J*32):NEXT
400 A=USR0(0)
420 GOTO400
999 GOTO999
2000 'ORG$7000:CONST PLANE=$7E80,HPAT=$7E00
2005 'VAR2,POS,SHX,SHX1,SHY,SPD,SHP,OFS,COR
2010 'VAR1,UP,DOWN,LEFT,RIGHT,FIRE,DIR,CLK
2020 '@PUT LDXSHP:LDU#PLANE:LDADIR:BEQPT2:LEAU48,U:@PT2 LDY#4:@PLP LDD,U:EORA,X:EORB1,X:STD,X:LDD2,U:EORA2,X:EORB32,X:STA2,X:STB32,X:LDD4,U:EORA33,X:EORB34,X:STD33,X:LDD6,U:EORA64,X:EORB65,X:STD64,X:LDD8,U:EORA66,X:EORB96,X:STA66,X:STB96,X
2025 'LDD10,U:EORA97,X:EORB98,X:STD97,X:LEAU12,U:LEAX128,X:LEAY-1,Y:BNEPLP:RTS
2200 '@KEYS LDX#FIRE+1:CLR,-X:CLR,-X:CLR,-X:CLR,-X:CLR,-X:LDD#$F703:@KLOOP BSRKCHK:LSLA:ORA#1:DECB:BNEKLOOP:RTS:@KCHK PSHSA:STA$FF02:LDA$FF00:BITA#$20:BNEKNOT:INC,X+:PULSA,PC:@KNOT LEAX1,X:PULSA,PC
2220 '@SMOV BSRKEYS:LDDSPD:TSTUP:BEQNUP:CLRDIR:ADDD#16:CMPD#$0400:BLENUP:LDD#$0400:@NUP TSTDOWN:BEQNDOW:CLRDIR:INCDIR:SUBD#16:CMPD#$FC00:BGENDOW:LDD#$FC00:@NDOW STDSPD:BMISM2:ADDDSHX+1:STDSHX+1:BCCNUP1:INCSHX
2225 '@NUP1 LDDSHP:TSTDIR:BNESM5:CMPD#$1600:BHSSM6:ADDD#$20:INCOFS+1:STDSHP:@SM6 RTS:@SM5 CMPD#$1000:BLSSM6:SUBD#$20:DECOFS+1:STDSHP:RTS
2230 '@SM2 ADDDSHX+1:STDSHX+1:BCSNUP1:DECSHX:BRANUP1
3000 '@HPT CLRA:STA-128,X:STA-96,X:STA-64,X:STA-32,X:LDD,U:STA,X:STB32,X:LDD2,U:STA64,X:STB96,X:LEAX256,X:LDD4,U:STA-128,X:STB-96,X:LDD6,U:STA-64,X:STB-32,X:CLRA:STA,X:STA32,X:STA64,X:STA96,X:@HEND RTS
3020 '@HDO LDDSHX:SUBDOFS:STDCOR:LDBCOR+1:CMPB#240:BHIHEND:LDA#32:MUL:LDX#$405:LEAXD,X:LDU#HPAT:LDACLK:ADDA#4:CMPA#48:BLOHDO2:CLRA:@HDO2 STACLK:ANDA#$F8:LEAUA,U:BSRHPT:LEAX-231,X:BRAHPT
4000 'DEFUSR0:LDX#0:STXSHX:STXSHX1:STXSPD:LDX#$9000:STXSHY:CLR DIR:LDX#$1611:STXSHP:LDD#128:STDOFS
4020 'LBSRPUT:@MLOOP SYNC:LBSRPUT:LBSRSMOV:LBSRPUT:BSRHDO:BRAMLOOP

GTAsm was hand written in machine code by Gwilym Thomas and is copyright © 1985. It probably hasn't been debugged since then so don't expect it to be free of errors. But do feel free to have fun with it developing 6809 assembly language programs for your Dragon 32.