# Compiler Syntax ## Comments Comments begin with a semicolon (;). A colon (:) following a label is optional ``` lda #$00 ;hi there label1: jmp label2 label2 beq label1 ``` ## Numbers and expressions Hexadecimal numbers begin with '$' or end with 'h'. Binary numbers begin with '%' or end with 'b'. Characters and strings are surrounded by single or double quotes. The characters (' " \\) within quotes must be preceded by a backslash (\\). ``` 12345 '12345' $ABCD 0ABCDh %01010101 01010101b ``` Supported operators (listed by precedence): ``` ( ) (unary) + - ~ ! < > * / % + - << >> < > <= >= = == != <> & ^ | && || ``` '=' and '<>' are equivalent to C's '==' and '!=' operators. The unary '<' and '>' operators give the lower and upper byte of a 16-bit word (respectively). All other operators function like their C equivalents. ## Labels Labels are case sensitive. The special '$' label holds the current program address. Labels beginning with '@' are local labels. They have limited scope, visible only between non-local labels. Names of local labels may be reused. ```asm6502 label1: @tmp1: @tmp2: label2: @tmp1: @tmp2: ``` Labels beginning with one or more '+' or '-' characters are nameless labels, especially useful for forward and reverse branches. ``` -- ldx #0 - lda $2002 ;loop (wait for vblank) bne - - lda $2002 ;nameless labels are easy to reuse.. bne - cpx #69 beq + ;forward branch.. cpx #96 beq +here ;use more characters to make more unique jmp -- ;multiple --'s handy for nested loops + ldx #0 +here nop ``` ## Assembler directives (in no specific order) All directives are case insensitive and can also be preceded by a period (.) `EQU` For literal string replacement, similar to #define in C. ``` one EQU 1 plus EQU + DB one plus one ;DB 1 + 1 ``` `=`\ \ Unlike EQU, statements with '=' are evaluated to a number first. Also unlike EQU, symbols created with '=' can be reused. ``` i=1 j EQU i+1 k=i+1 ;k=1+1 i=j+1 ;i=i+1+1 i=k+1 ;i=2+1 ``` `INCLUDE (also INCSRC)` Assemble another source file as if it were part of the current source. ``` INCLUDE whatever.asm ``` `INCBIN (also BIN)` Add the contents of a file to the assembly output. ``` moredata: INCBIN whatever.bin ``` An optional file offset and size can be specified. ``` INCBIN foo.bin, $400 ;read foo.bin from $400 to EOF INCBIN foo.bin, $200, $2000 ;read $2000 bytes, starting from $200 ``` `DB, DW (also BYTE/WORD, DCB/DCW, DC.B/DC.W)` Emit byte(s) or word(s). Multiple arguments are separated by commas. Strings can be "shifted" by adding a value to them. ``` DB $01,$02,$04,$08 DB "ABCDE"+1 ;equivalent to DB "BCDEF" DB "ABCDE"-"A"+32 ;equivalent to DB 32,33,34,35,36 ``` `DL, DH` Similar to DB, outputting only the LSB or MSB of a value. ``` DL a,b,c,d ;equivalent to DB a, >b, >c, >d ``` `HEX` Compact way of laying out a table of hex values. Only raw hex values are allowed, no expressions. Spaces can be used to separate numbers. ``` HEX 456789ABCDEF ;equivalent to DB $45,$67,$89,$AB,$CD,$EF HEX 0 1 23 4567 ;equivalent to DB $00,$01,$23,$45,$67 ``` `DSB, DSW (also DS.B/DS.W)` Define storage (bytes or words). The size argument may be followed by a fill value (default filler is 0). ``` DSB 4 ;equivalent to DB 0,0,0,0 DSB 8,1 ;equivalent to DB 1,1,1,1,1,1,1,1 DSW 4,$ABCD ;equivalent to DW $ABCD,$ABCD,$ABCD,$ABCD ``` `PAD` Fill memory from the current address to a specified address. A fill value may also be specified. ``` PAD $FFFA ;equivalent to DSB $FFFA-$ PAD $FFFA,$EA ;equivalent to DSB $FFFA-$,$EA ``` `ORG` Set the starting address if it hasn't been assigned yet, otherwise ORG functions like PAD. ``` ORG $E000 ;start assembling at $E000 . . . ORG $FFFA,$80 ;equivalent to PAD $FFFA,$80 ``` `ALIGN` Fill memory from the current address to an N byte boundary. A fill value may also be specified. ``` ALIGN 256,$EA ``` `FILLVALUE` Change the default filler for PAD, ALIGN, etc. ``` FILLVALUE $FF ``` `BASE` Set the program address. This is useful for relocatable code, multiple code banks, etc. The same can also be accomplished by assigning the '$' symbol directly (i.e. '$=9999'). ``` oldaddr=$ BASE $6000 stuff: . . . BASE oldaddr+$-stuff ``` `IF / ELSEIF / ELSE / ENDIF` Process a block of code if an expression is true (nonzero). ``` IF j>0 DB i/j ELSE DB 0 ENDIF ``` `IFDEF / IFNDEF` Process a block of code if a symbol has been defined / not defined. ``` IFDEF _DEBUG_ . . . ENDIF ``` `MACRO / ENDM` MACRO name args... Define a macro. Macro arguments are comma separated. Labels defined inside macros are local (visible only to that macro) ``` MACRO setAXY x,y,z LDA #x LDX #y LDY #z ENDM setAXY $12,$34,$56 ;expands to LDA #$12 ; LDX #$34 ; LDY #$56 ``` `REPT / ENDR`\ Repeat a block of code a specified number of times. Labels defined inside REPT are local. ``` i=0 REPT 256 DB i i=i+1 ENDR ``` `ENUM / ENDE` Reassign PC and suppress assembly output. Useful for defining variables in RAM. ``` ENUM $200 foo: db 0 foo2: db 0 ENDE ``` `ERROR` Stop assembly and display a message. ``` IF x>100 ERROR "X is out of range :(" ENDIF ``` The information on this page is provided from the asm6f documentation by loopy.