rgbasmlanguage documentation


This is the full description of the language used by rgbasm(1). The description of the instructions supported by the GameBoy CPU is in gbz80(7).



The syntax is line‐based, just as in any other assembler, meaning that you do one instruction or pseudo‐op per line:
[label] [instruction] [;comment]
John: ld a,87 ;Weee
All pseudo‐ops, mnemonics and registers (reserved keywords) are case‐insensitive and all labels are case‐sensitive.


Before you can start writing code, you must define a section. This tells the assembler what kind of information follows and, if it is code, where to put it.
This switches to the section called "CoolStuff" (or creates it if it doesn't already exist) and it defines it as a code section. All sections assembled at the same time that have the same name, type, etc, are considered to be the same one, and their code is put together in the object file generated by the assembler. All other sections must have a unique name, even in different source files, or the linker will treat it as an error.
Possible section types are as follows:
A ROM section. Mapped to memory at $0000–$3FFF (or $0000-$7FFF if tiny ROM mode is enabled in rgblink(1)).
A banked ROM section. Mapped to memory at $4000–$7FFF. Valid banks range from 1 to 511. Not available if tiny ROM mode is enabled in rgblink(1).
A banked video RAM section. Mapped to memory at $8000–$9FFF. Can only allocate memory, not fill it. Valid banks are 0 and 1 but bank 1 isn't available if DMG mode is enabled in rgblink(1).
A banked external (save) RAM section. Mapped to memory at $A000–$BFFF. Can only allocate memory, not fill it. Valid banks range from 0 to 15.
A general-purpose RAM section. Mapped to memory at $C000–$CFFF, or $C000-$DFFF if DMG mode is enabled in rgblink(1). Can only allocate memory, not fill it.
A banked general-purpose RAM section. Mapped to memory at $D000–$DFFF. Can only allocate memory, not fill it. Valid banks range from 1 to 7. Not available if DMG mode is enabled in rgblink(1).
An object attributes RAM section. Mapped to memory at $FE00-$FE9F. Can only allocate memory, not fill it.
A high RAM section. Mapped to memory at $FF80–$FFFE. Can only allocate memory, not fill it.
NOTE: If you use this method of allocating HRAM the assembler will NOT choose the short addressing mode in the LD instructions LD [$FF00+n8],A and LD A,[$FF00+n8] because the actual address calculation is done by the linker. If you find this undesirable you can use RSSET / RB / RW instead or use the LDH [$FF00+n8],A and LDH A,[$FF00+n8] syntax instead. This forces the assembler to emit the correct instruction and the linker to check if the value is in the correct range.
A section is usually defined as a floating one, but the code can restrict where the linker can place it.
If a section is defined with no indications, it is a floating section. The linker will decide where to place it in the final binary and it has no obligation to follow any specific rules. The following example defines a section that can be placed anywhere in any ROMX bank:
If it is needed, the following syntax can be used to fix the base address of the section:
SECTION "CoolStuff",ROMX[$4567]
It won't, however, fix the bank number, which is left to the linker. If you also want to specify the bank you can do:
SECTION "CoolStuff",ROMX[$4567],BANK[3]
And if you only want to force the section into a certain bank, and not it's position within the bank, that's also possible:
In addition, you can specify byte alignment for a section. This ensures that the section starts at a memory address where the given number of least-significant bits are 0. This can be used along with BANK, if desired. However, if an alignment is specified, the base address must be left unassigned. This can be useful when using DMA to copy data or when it is needed to align the start of an array to 256 bytes to optimize the code that accesses it.
SECTION "OAM Data",WRAM0,ALIGN[8]; align to 256 bytes
SECTION "VRAM Data",ROMX,BANK[2],ALIGN[4]; align to 16 bytes
HINT: If you think this is a lot of typing for doing a simple ORG type thing you can quite easily write an intelligent macro (called ORG for example) that uses @ for the section name and determines correct section type etc as arguments for SECTION.
POPS and PUSHS provide the interface to the section stack. PUSHS will push the current section context on the section stack. POPS can then later be used to restore it. Useful for defining sections in included files when you don't want to destroy the section context for the program that included your file. The number of entries in the stack is limited only by the amount of memory in your machine.
Sections can also be placed by using a linkerscript file. The format is described in rgblink(5). They allow the user to place floating sections in the desired bank in the order specified in the script. This is useful if the sections can't be placed at an address manually because the size may change, but they have to be together.



RGBDS supports several types of symbols:
Used to assign a memory location with a name
Give a constant a name.
Almost the same as EQUate, but you can change the value of a SET during assembling.
Structure (the RS group)
Define a structure easily.
String equate (EQUS)
Give a frequently used string a name. Can also be used as a mini-macro, like #define in C.
A block of code or pseudo instructions that you invoke like any other mnemonic. You can give them arguments too.
A symbol cannot have the same name as a reserved keyword.
One of the assembler's main tasks is to keep track of addresses for you so you don't have to remember obscure numbers but can make do with a meaningful name, a label.
This can be done in a number of ways:
ThisWillBeExported:: ;note the two colons
In the line where a label is defined there musn't be any whitespace before it. Local labels are only accessible within the scope they are defined. A scope starts after a global label and ends at the next global label. Declaring a normal label with :: does an EXPORT at the same time.
Labels will normally change their value during the link process and are thus not constant. The exception is the case in which the base address of a section is fixed, so the address of the label is known at assembly time.
The subtraction of two labels is only constant (known at assembly time) if they are two local labels that belong to the same scope, or they are two global labels that belong to sections with fixed base addresses.
EQUates are constant symbols. They can, for example, be used for things such as bit-definitions of hardware registers.
Note that a colon (:) following the label-name is not allowed. EQUates cannot be exported and imported. They don't change their value during the link process.
SETs are similar to EQUates. They are also constant symbols in the sense that their values are defined during the assembly process. These symbols are normally used in macros.
COUNT      SET 2 
Note that a colon (:) following the label-name is not allowed. SETs cannot be exported and imported. Alternatively you can use = as a synonym for SET.
The RS group of commands is a handy way of defining structures:
str_pStuff    RW   1 
str_tData     RB   256 
str_bCount    RB   1 
str_SIZEOF    RB   0
The example defines four equated symbols:
str_pStuff = 0 
str_tData  = 2 
str_bCount = 258 
str_SIZEOF = 259
There are four commands in the RS group of commands:
Command Meaning
RSRESET Resets the _RS counter to zero.
RSSET constexpr Sets the _RS counter to constexpr.
RB constexpr Sets the preceding symbol to _RS and adds constexpr to _RS.
RW constexpr Sets the preceding symbol to _RS and adds constexpr * 2 to _RS.
Note that a colon (:) following the symbol-name is not allowed. RS symbols cannot be exported and imported. They don't change their value during the link process.
EQUS is used to define string-symbols. Wherever the assembler meets a string symbol its name is replaced with its value. If you are familiar with C you can think of it as the same as #define.
Note that : following the label-name is not allowed, and that strings must be quoted to be useful.
This will be interpreted as:
ld a,[hl+]
db "John"
String-symbols can also be used to define small one-line macros:
PUSHA EQUS "push af\npush bc\npush de\npush hl\n"
Note that a colon (:) following the label-name is not allowed. String equates can't be exported or imported.
Important note: An EQUS can be expanded to a string that contains another EQUS and it will be expanded as well. This means that, if you aren't careful, you may trap the assembler into an infinite loop if there's a circular dependency in the expansions. Also, a MACRO can have inside an EQUS which references the same MACRO, which has the same problem.
One of the best features of an assembler is the ability to write macros for it. Macros also provide a method of passing arguments to them and they can then react to the input using IF-constructs.
MyMacro: MACRO 
         ld   a,80 
         call MyFunc 
Note that a colon (:) following the macro-name is required. Macros can't be exported or imported. It's valid to call a macro from a macro (yes, even the same one).
The above example is a very simple macro. You execute the macro by typing its name.
         add  a,b 
         ld   sp,hl 
         MyMacro ;This will be expanded 
         sub  a,87
When the assembler meets MyMacro it will insert the macrodefinition (the text enclosed in MACRO / ENDM).
Suppose your macro contains a loop.
LoopyMacro: MACRO 
            xor  a,a 
.loop       ld   [hl+],a 
            dec  c 
            jr   nz,.loop 
This is fine. That is, if you only use the macro once per scope. To get around this problem there is a special label string equate called \@ that you can append to your labels and it will then expand to a unique string.
\@ also works in REPT-blocks should you have any loops there.
LoopyMacro: MACRO 
            xor  a,a 
.loop\@     ld   [hl+],a 
            dec  c 
            jr   nz,.loop\@ 
Important note: Since a MACRO can call itself (or a different MACRO that calls the first one) there can be problems of circular dependency. They trap the assembler in an infinite loop, so you have to be careful when using recursion with MACROs. Also, a MACRO can have inside an EQUS which references the same MACRO, which has the same problem.
Macro Arguments
I'd like LoopyMacro a lot better if I didn't have to pre-load the registers with values and then call it. What I'd like is the ability to pass it arguments and it then loaded the registers itself.
And I can do that. In macros you can get the arguments by using the special macro string equates \1 through \9, \1 being the first argument specified on the calling of the macro.
LoopyMacro: MACRO 
            ld   hl,\1 
            ld   c,\2 
            xor  a,a 
.loop\@     ld   [hl+],a 
            dec  c 
            jr   nz,.loop\@ 
Now I can call the macro specifying two arguments. The first being the address and the second being a bytecount. The macro will then reset all bytes in this range.
LoopyMacro MyVars,54
Arguments are passed as string equates. There's no need to enclose them in quotes. An expression will not be evaluated first but passed directly. This means that it's probably a very good idea to use brackets around \1 to \9 if you perform further calculations on them. For instance, if you pass 1 + 2 as the first argument and then do PRINTV \1 * 2 you will get the value 5 on screen and not 6 as you might have expected.
In reality, up to 256 arguments can be passed to a macro, but you can only use the first 9 like this. If you want to use the rest, you need to use the keyword SHIFT.
SHIFT is a special command only available in macros. Very useful in REPT-blocks. It will "shift" the arguments by one "to the left". \1 will get the value of \2, \2 will get the value in \3 and so forth.
This is the only way of accessing the value of arguments from 10 to 256.

Exporting and importing symbols

Importing and exporting of symbols is a feature that is very useful when your project spans many source-files and, for example, you need to jump to a routine defined in another file.
Exporting of symbols has to be done manually, importing is done automatically if the assembler doesn't know where a symbol is defined.
EXPORT label [, label , ...]
The assembler will make label accessible to other files during the link process.
GLOBAL label [, label , ...]
If label is defined during the assembly it will be exported, if not, it will be imported. Handy (very!) for include-files. Note that, since importing is done automatically, this keyword has the same effect as EXPORT.

Purging symbols

PURGE allows you to completely remove a symbol from the symbol table as if it had never existed. USE WITH EXTREME CAUTION!!! I can't stress this enough, you seriously need to know what you are doing. DON'T purge symbol that you use in expressions the linker needs to calculate. In fact, it's probably not even safe to purge anything other than string symbols and macros.
Kamikaze EQUS  "I don't want to live anymore" 
AOLer    EQUS  "Me too" 
         PURGE Kamikaze, AOLer
Note that string symbols that are part of a PURGE command WILL NOT BE EXPANDED as the ONLY exception to this rule.

Predeclared Symbols

The following symbols are defined by the assembler:
Type Name Contents
EQU @ PC value
EQU _PI Fixed point π
SET _RS _RS Counter
EQU _NARG Number of arguments passed to macro
EQU __LINE__ The current line number
EQUS __FILE__ The current filename
EQUS __DATE__ Today's date
EQUS __TIME__ The current time
EQUS __ISO_8601_LOCAL__ ISO 8601 timestamp (local)
EQUS __ISO_8601_UTC__ ISO 8601 timestamp (UTC)
EQU __UTC_YEAR__ Today's year
EQU __UTC_MONTH__ Today's month number, 1-12
EQU __UTC_DAY__ Today's day of the month, 1-31
EQU __UTC_HOUR__ Current hour, 0-23
EQU __UTC_MINUTE__ Current minute, 0-59
EQU __UTC_SECOND__ Current second, 0-59


Defining constant data

DB defines a list of bytes that will be stored in the final image. Ideal for tables and text.
DB 1,2,3,4,"This is a string"
Alternatively, you can use DW to store a list of words. Strings are not allowed as arguments to DW.
You can also use DB and DW without arguments. This works exactly like DS 1 and DS 2 respectively. Consequently, DB and DW can be used in a WRAM0 / WRAMX / HRAM / VRAM / SRAM section.

Declaring variables in a RAM section

DS allocates a number of bytes. The content is undefined. This is the preferred method of allocationg space in a RAM section. You can, however, use DB and DW without any arguments instead.
DS str_SIZEOF ;allocate str_SIZEOF bytes

Including binary files

You probably have some graphics you'd like to include. Use INCBIN to include a raw binary file as it is. If the file isn't found in the current directory, the include-path list passed to the linker on the command line will be searched.
INCBIN "titlepic.bin"
INCBIN "sprites/hero.bin" ; UNIX
INCBIN "sprites\\hero.bin" ; Windows
You can also include only part of a file with INCBIN. The example below includes 256 bytes from data.bin starting from byte 78.
INCBIN "data.bin",78,256


Printing things during assembly

These three instructions type text and values to stdout. Useful for debugging macros or wherever you may feel the need to tell yourself some important information.
PRINTT "I'm the greatest programmer in the whole wide world\n" 
PRINTV (2+3)/5 
PRINTF MUL(3.14,3987.0)
prints out a string.
prints out an integer value or, as in the example, the result of a calculation. Unsurprisingly, you can also print out a constant symbols value.
prints out a fixed point value.

Automatically repeating blocks of code

Suppose you're feeling lazy and you want to unroll a time consuming loop. REPT is here for that purpose. Everything between REPT and ENDR will be repeated a number of times just as if you done a copy/paste operation yourself. The following example will assemble add a,c four times:
add  a,c 
You can also use REPT to generate tables on the fly:
; -- 
; -- Generate a 256 byte sine table with values between 0 and 128 
; -- 
ANGLE SET   0.0 
      REPT  256 
      DB    (MUL(64.0,SIN(ANGLE))+64.0)>>16 
REPT is also very useful in recursive macros and, as in macros, you can also use the special label operator \@. REPT-blocks can be nested.

Aborting the assembly process

FAIL and WARN can be used to print errors and warnings respectively during the assembly process. This is especially useful for macros that get an invalid argument. FAIL and WARN take a string as the only argument and they will print this string out as a normal error with a line number.
FAIL stops assembling immediately while WARN shows the message but continues afterwards.

Including other source files

Use INCLUDE to process another assembler-file and then return to the current file when done. If the file isn't found in the current directory the include-path list will be searched. You may nest INCLUDE calls infinitely (or until you run out of memory, whichever comes first).

Conditional assembling

The three commands IF, ELSE and ENDC are used to conditionally assemble parts of your file. This is a powerful feature commonly used in macros.
IF 2+2==4 
PRINTT "2+2==4\n" 
PRINTT "2+2!=4\n" 
The ELSE block is optional. IF / ELSE / ENDC blocks can be nested.

Integer and Boolean expressions

An expression can be composed of many things. Expressions are always evaluated using signed 32-bit math.
The most basic expression is just a single number.
Numeric Formats
There are a number of numeric formats.
  • Hexadecimal: $0123456789ABCDEF. Case-insensitive
  • Decimal: 0123456789
  • Octal: &01234567
  • Binary: %01
  • Fixedpoint (16.16): 01234.56789
  • Character constant: "ABYZ"
  • Gameboy graphics: `0123
The last one, Gameboy graphics, is quite interesting and useful. The values are actually pixel values and it converts the “chunky” data to “planar” data as used in the Gameboy.
DW `01012323
Admittedly, an expression with just a single number is quite boring. To spice things up a bit there are a few operators you can use to perform calculations between numbers.
A great number of operators you can use in expressions are available (listed in order of precedence):
Operator Meaning
() Precedence override
FUNC() Function call
~ + - Unary not/plus/minus
* / % Multiply/divide/modulo
<< >> Shift left/right
& | ^ Binary and/or/xor
+ - Add/subtract
!= == <= Boolean comparison
>= < > Boolean comparison (Same precedence as the others)
&& || Boolean and/or
! Unary Boolean not
The result of the boolean operators is zero if when FALSE and non-zero when TRUE. It is legal to use an integer as the condition for IF blocks. You can use symbols instead of numbers in your expression if you wish.
An expression is said to be constant when it doesn't change its value during linking. This basically means that you can't use labels in those expressions. The instructions in the macro-language all require expressions that are constant. The only exception is the subtraction of labels in the same section or labels that belong to sections with a fixed base addresses, all of which must be defined in the same source file (the calculation cannot be passed to the object file generated by the assembler). In this case, the result is a constant that can be calculated at assembly time.

Fixed‐point Expressions

Fixed point constants are basically normal 32-bit constants where the upper 16 bits are used for the integer part and the lower 16 bits are used for the fraction (65536ths). This means that you can use them in normal integer expression, and some integer operators like plus and minus don't care whether the operands are integer or fixed-point. You can easily convert a fixed-point number to an integer by shifting it right 16 bits. It follows that you can convert an integer to a fixed-point number by shifting it left.
Some things are different for fixed-point math, though, which is why you have the following functions to use:
Name Operation
DIV(x,y) x/y
MUL(x,y) x*y
SIN(x) sin(x)
COS(x) cos(x)
TAN(x) tan(x)
ASIN(x) arcsin(x)
ACOS(x) arccos(x)
ATAN(x) arctan(x)
ATAN2(x,y) Angle between (x,y) and (1,0)
These functions are extremely useful for automatic generation of various tables. A circle has 65536.0 degrees. Sine values are between [-1.0; 1.0].
; -- 
; -- Generate a 256 byte sine table with values between 0 and 128 
; -- 
ANGLE SET   0.0 
      REPT  256 
      DB    (MUL(64.0,SIN(ANGLE))+64.0)>>16 

String Expressions

The most basic string expression is any number of characters contained in double quotes ("for instance"). Like in C, the escape character is \, and there are a number of commands you can use within a string:
String Meaning
\\ Backslash
\" Double quote
\, Comma
\{ Curly bracket left
\} Curly bracket right
\n Newline ($0A)
\t Tab ($09)
\1 - \9 Macro argument (Only the body of a macros)
\@ Label name suffix (Only in the body of macros and repts)
A funky feature is {symbol} withing a string. This will examine the type of the symbol and insert its value accordingly. If symbol is a string symbol, the symbols value is simply copied. If it's a numeric symbol, the value is converted to hexadecimal notation and inserted as a string.
HINT: The {symbol} construct can also be used outside strings. The symbol's value is again inserted as a string. This is just a short way of doing "{symbol}".
Whenever the macro-language expects a string you can actually use a string expression. This consists of one or more of these function (yes, you can nest them). Note that some of these functions actually return an integer and can be used as part of an integer expression!
Name Operation
STRLEN(string) Returns the number of characters in string
STRCAT(str1,str2) Appends str2 to str1.
STRCMP(str1,str2) Returns negative if str1 is alphabetically lower than str2, zero if they match, positive if str1 is greater than str2.
STRIN(str1,str2) Returns the position of str2 in str1 or zero if it's not present (first character is position 1).
STRSUB(str,pos,len) Returns a substring from str starting at pos (first character is position 1) and with len characters.
STRUPR(str) Converts all characters in str to capitals and returns the new string.
STRLWR(str) Converts all characters in str to lower case and returns the new string.

Other functions

There are a few other functions that do various useful things:
Name Operation
BANK(label) Returns the bank number label is in. The linker will have to resolve this so it can't be used when the expression has to be constant.
DEF(label) Returns TRUE if label has been defined.
HIGH(r16/cnst/lbl) Returns the top 8 bits of the operand if it is a label or constant, or the top 8-bit register if it is a 16-bit register.
LOW(r16/cnst/lbl) Returns the bottom 8 bits of the operand if it is a label or constant, or the bottom 8-bit register if it is a 16-bit register (AF isn't a valid register for this function).


Changing options while assembling

OPT can be used to change some of the options during assembling the source instead of defining them on the commandline.
OPT takes a comma-seperated list of options as its argument:
OPT   g.oOX ;Set the GB graphics constants to use these characters 
DW    `..ooOOXX 
DW    `00112233
The options that OPT can modify are currently: b, e and g.
POPO and PUSHO provide the interface to the option stack. PUSHO will push the current set of options on the option stack. POPO can then later be used to restore them. Useful if you want to change some options in an include file and you don't want to destroy the options set by the program that included your file. The stacks number of entries is limited only by the amount of memory in your machine.




rgbasm(1), rgblink(1), rgblink(5), rgbds(5), rgbds(7), gbz80(7)


rgbds was originally written by Carsten Sørensen as part of the ASMotor package, and was later packaged in RGBDS by Justin Lloyd. It is now maintained by a number of contributors at