ref: 03fe077b41ae81d240e2c467bdf7275d57b3c462
dir: /src/asm/rgbasm.5/
.\"
.\" This file is part of RGBDS.
.\"
.\" Copyright (c) 2017-2018, Antonio Nino Diaz and RGBDS contributors.
.\"
.\" SPDX-License-Identifier: MIT
.\"
.Dd December 5, 2019
.Dt RGBASM 5
.Os
.Sh NAME
.Nm rgbasm
.Nd language documentation
.Sh DESCRIPTION
This is the full description of the language used by
.Xr rgbasm 1 .
The description of the instructions supported by the Game Boy CPU is in
.Xr gbz80 7 .
.Pp
.Sh GENERAL
.Ss Syntax
The syntax is line‐based, just as in any other assembler, meaning that you do one instruction or pseudo‐op per line:
.Pp
.Dl Oo Ar label Oc Oo Ar instruction Oc Oo Ar ;\ comment Oc
.Pp
Example:
.Pp
.Bd -literal -offset indent
John: ld a,87 ;Weee
.Ed
.Pp
All pseudo‐ops, mnemonics and registers (reserved keywords) are case‐insensitive
and all labels are case‐sensitive.
.Pp
There are two syntaxes for comments.
In both cases, a comment ends at the end of the line.
The most common one is: anything that follows a semicolon
.Ql \&;
that isn't inside a string is a comment.
There is another format: anything that follows a
.Ql *
that is placed right at the start of a line is a comment.
The assembler removes all comments from the code before doing anything else.
.Pp
Sometimes lines can be too long and it may be necessary to split them.
The syntax to do so is the following one:
.Pp
.Bd -literal -offset indent
DB 1, 2, 3, 4 \[rs]
5, 6, 7, 8
.Ed
.Pp
This works anywhere in the code except inside of strings.
To split strings it is needed to use
.Fn STRCAT
like this:
.Pp
.Bd -literal -offset indent
db STRCAT("Hello ", \[rs]
"world!")
.Ed
.Pp
.Ss Sections
.Ic SECTION Ar name , type
.Pp
.Ic SECTION Ar name , type , options
.Pp
.Ic SECTION Ar name , type Ns Bo Ar addr Bc
.Pp
.Ic SECTION Ar name , type Ns Bo Ar addr Bc , Ar options
.Pp
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.
.Pp
.Ar name
is a string enclosed in double quotes and can be a new name or the name of an existing section.
All sections assembled at the same time that have the same name are considered to be the same section, and their code is put together in the object file generated by the assembler.
If the type doesn't match, an error occurs.
All other sections must have a unique name, even in different source files, or the linker will treat it as an error.
.Pp
Possible section
.Ar type Ns s
are as follows:
.Pp
.Bl -tag
.It Cm ROM0
A ROM section.
.Ar addr
can range from
.Ad $0000
to
.Ad $3FFF ,
or
.Ad $0000
to
.Ad $7FFF
if tiny ROM mode is enabled in
.Xr rgblink 1 .
.It Cm ROMX
A banked ROM section.
.Ar addr
can range from
.Ad $4000
to
.Ad $7FFF .
.Ar bank
can range from 1 to 511.
Not available if tiny ROM mode is enabled in
.Xr rgblink 1 .
.It Cm VRAM
A banked video RAM section.
.Ar addr
can range from
.Ad $8000
to
.Ad $9FFF .
.Ar bank
can be 0 or 1 but bank 1 is unavailable if DMG mode is enabled in
.Xr rgblink 1 .
Memory in this section can only be allocated with
.Sy DS ,
not filled with data.
.It Cm SRAM
A banked external (save) RAM section.
.Ar addr
can range from
.Ad $A000
to
.Ad $BFFF .
.Ar bank
can range from 0 to 15.
Memory in this section can only be allocated with
.Sy DS ,
not filled with data.
.It Cm WRAM0
A general-purpose RAM section.
.Ar addr
can range from
.Ad $C000
to
.Ad $CFFF ,
or
.Ad $C000
to
.Ad $DFFF
if DMG mode is enabled in
.Xr rgblink 1 .
Memory in this section can only be allocated with
.Sy DS ,
not filled with data.
.It Cm WRAMX
A banked general-purpose RAM section.
.Ar addr
can range from
.Ad $D000
to
.Ad $DFFF .
.Ar bank
can range from 1 to 7.
Not available if DMG mode is enabled in
.Xr rgblink 1 .
Memory in this section can only be allocated with
.Sy DS ,
not filled with data.
.It Cm OAM
An object attribute RAM section.
.Ar addr
can range from
.Ad $FE00
to
.Ad $FE9F .
Memory in this section can only be allocated with
.Sy DS ,
not filled with data.
.It Cm HRAM
A high RAM section.
.Ar addr
can range from
.Ad $FF80
to
.Ad $FFFE.
Memory in this section can only be allocated with
.Sy DS ,
not filled with data.
.Pp
.Sy Note :
If you use this method of allocating HRAM the assembler will
.Em not
choose the short addressing mode in the LD instructions
.Ic ld [$FF00+n8],A
and
.Ic ld A,[$FF00+n8]
because the actual address calculation is done by the linker.
If you find this undesirable you can use
.Ic RSSET , RB ,
or
.Ic RW
instead or use the
.Sy ldh [$FF00+n8],A
and
.Sy 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.
.El
.Pp
.Ar option Ns s are comma-separated and may include:
.Bl -tag
.It Cm BANK Ns Bq Ar bank
Specify which
.Ar bank
for the linker to place the section.
.It Cm ALIGN Ns Bq Ar align
Place the section at an address whose
.Ar align
least‐significant bits are zero.
It is a syntax error to use this option with
.Ar addr .
.El
.Pp
If
.Bq Ar addr
is not specified, the section is considered
.Dq floating ;
the linker will automatically calculate an appropriate address for the section.
Similarly, if
.Cm BANK Ns Bq Ar bank
is not specified, the linker will automatically find a bank with enough space.
.Pp
Sections can also be placed by using a linker script file.
The format is described in
.Xr 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.
.Pp
Section examples:
.Bd -literal -offset indent
SECTION "CoolStuff",ROMX
.Ed
.Pp
This switches to the section called
.Dq CoolStuff
(or creates it if it doesn't already exist) and defines it as a code section.
.Pp
The following example defines a section that can be placed anywhere in any ROMX
bank:
.Pp
.Bd -literal -offset indent
SECTION "CoolStuff",ROMX
.Ed
.Pp
If it is needed, the the base address of the section can be specified:
.Pp
.Bd -literal -offset indent
SECTION "CoolStuff",ROMX[$4567]
.Ed
.Pp
An example with a fixed bank:
.Pp
.Bd -literal -offset indent
SECTION "CoolStuff",ROMX[$4567],BANK[3]
.Ed
.Pp
And if you only want to force the section into a certain bank, and not its position within the bank, that's also possible:
.Pp
.Bd -literal -offset indent
SECTION "CoolStuff",ROMX,BANK[7]
.Ed
.Pp
Alignment examples:
one use could be 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.
.Pp
.Bd -literal -offset indent
SECTION "OAM Data",WRAM0,ALIGN[8] ; align to 256 bytes
SECTION "VRAM Data",ROMX,BANK[2],ALIGN[4] ; align to 16 bytes
.Ed
.Pp
.Sy Hint :
If you think this is a lot of typing for doing a simple
.Dq org
type thing you can quite easily write an intelligent macro (called
.Ic ORG
for example) that uses
.Ic \[rs]@
for the section name, and determines the correct section type etc. as arguments for
.Ic SECTION .
.Ss Section Stack
.Ic POPS
and
.Ic PUSHS
provide the interface to the section stack.
.Pp
.Ic PUSHS
will push the current section context on the section stack.
.Ic 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.
.Ss RAM Code
Sometimes you want to have some code in RAM.
But then you can't simply put it in a RAM section, you have to store it in ROM and copy it to RAM at some time.
.Pp
This means the code (or data) will not be stored in the place it gets executed.
Luckily,
.Ic LOAD
blocks are the perfect solution to that.
Here's an example of how to use them:
.Bd -literal -offset indent
SECTION "LOAD example", ROMX
CopyCode:
ld de, RAMCode
ld hl, RAMLocation
ld c, RAMLocation.end - RAMLocation
.loop
ld a, [de]
inc de
ld [hli], a
dec c
jr nz, .loop
ret
RAMCode:
LOAD "RAM code", WRAM0
RAMLocation:
ld hl, .string
ld de, $9864
.copy
ld a, [hli]
ld [de], a
inc de
and a
jr nz, .copy
ret
.string
db "Hello World!", 0
.end
ENDL
.Ed
.Pp
A
.Ic LOAD
block feels similar to a
.Ic SECTION
declaration because it creates a new one.
All data and code generated within such a block is placed in the current section like usual, but all labels are created as if the it was placed in this newly-created section.
.Pp
In the example above, all of the code and data will end up in the "LOAD example" section.
You will notice the
.Ic RAMCode
and
.Ic RAMLocation
labels.
The former is situated in ROM, where the code is stored, the latter in RAM, where the code will be loaded.
.Pp
You cannot nest
.Ic LOAD
blocks, nor can you change the current section within them.
.Sh SYMBOLS
.Pp
.Ss Symbols
RGBDS supports several types of symbols:
.Pp
.Bl -hang
.It Sy Label
Used to give a name to a memory location.
.It Sy EQUate
Give a constant a name.
.It Ic SET
Almost the same as EQUate, but you can change the value of a SET during assembling.
.It Sy Structure Pq Sy the RS group
Define a structure easily.
.It Sy String equate Pq Ic EQUS
Give a frequently used string a name.
Can also be used as a mini-macro, like
.Fd #define
in C.
.It Ic MACRO
A block of code or pseudo instructions that you invoke like any other mnemonic.
You can give them arguments too.
.El
.Pp
A symbol cannot have the same name as a reserved keyword.
.Bl -hang
.It Sy Label
.Pp
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.
.Pp
This can be done in a number of ways:
.Pp
.Bd -literal -offset indent
GlobalLabel
AnotherGlobal:
\&.locallabel
\&.yet_a_local:
AnotherGlobal.with_another_local:
ThisWillBeExported:: ;note the two colons
ThisWillBeExported.too::
.Ed
.Pp
.Em \&In the line where a label is defined there mustn'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 label (global or local) with :: does an
.Ic EXPORT
at the same time.
(See
.Sx Exporting and importing symbols
below).
Local labels can be declared as
.Ql scope.local
or simply as as
.Ql .local .
If the former notation is used, the scope must be the actual current scope.
.Pp
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.
.Pp
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.
.Pp
.It Ic EQU
.Pp
EQUates are constant symbols, and can be imported or exported.
They can, for example, be used for things such as bit definitions of hardware registers.
.Pp
.Bd -literal -offset indent
SCREEN_WIDTH equ 160 ; In pixels
SCREEN_HEIGHT equ 144
.Ed
.Pp
Note that a colon
.Ql \&:
following the name is not allowed.
They don't change their value during the link process.
.It Sy SET
.Pp
SETs are similar to EQUates, and can be imported and exported as well.
They are also constant symbols in the sense that their values are defined during the assembly process.
These symbols are typically used in macros.
.Pp
.Bd -literal -offset indent
ARRAY_SIZE EQU 4
COUNT SET 2
COUNT SET ARRAY_SIZE+COUNT
.Ed
.Pp
Note that a colon
.Ql \&:
following the name is not allowed.
Alternatively you can use
.Ql =
as a synonym for SET.
.Pp
.Bd -literal -offset indent
COUNT = 2
.Ed
.Pp
.It Sy RSSET , RSRESET , RB , RW
.Pp
The RS group of commands is a handy way of defining structures:
.Pp
.Bd -literal -offset indent
RSRESET
str_pStuff RW 1
str_tData RB 256
str_bCount RB 1
str_SIZEOF RB 0
.Ed
.Pp
The example defines four equated symbols:
.Pp
.Bd -literal -offset indent
str_pStuff = 0
str_tData = 2
str_bCount = 258
str_SIZEOF = 259
.Ed
.Pp
There are four commands in the RS group of commands:
.Pp
.Bl -column "RSSET constexpr"
.It Sy Command Ta Sy Meaning
.It Ic RSRESET Ta Resets the Ic _RS No counter to zero.
.It Ic RSSET Ar constexpr Ta Sets the Ic _RS No counter to Ar constexpr .
.It Ic RB Ar constexpr Ta Sets the preceding symbol to Ic _RS No and adds Ar constexpr No to Ic _RS .
.It Ic RW Ar constexpr Ta Sets the preceding symbol to Ic _RS No and adds Ar constexpr No * 2 to Ic _RS.
.It Ic RL Ar constexpr Ta Sets the preceding symbol to Ic _RS No and adds Ar constexpr No * 4 to Ic _RS.
(In practice, this one cannot be used due to a bug).
.El
.Pp
Note that a colon
.Ql \&:
following the name is not allowed.
.Sy RS
symbols can be exported and imported.
They don't change their value during the link process.
.Pp
.It Ic EQUS
.Pp
.Ic 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 similar to
.Fd #define .
.Pp
.Bd -literal -offset indent
COUNTREG EQUS "[hl+]"
ld a,COUNTREG
PLAYER_NAME EQUS "\[rs]"John\[rs]""
db PLAYER_NAME
.Ed
.Pp
This will be interpreted as:
.Pp
.Bd -literal -offset indent
ld a,[hl+]
db "John"
.Ed
.Pp
String symbols can also be used to define small one-line macros:
.Pp
.Bd -literal -offset indent
PUSHA EQUS "push af\[rs]npush bc\[rs]npush de\[rs]npush hl\[rs]n"
.Ed
.Pp
Note that a colon
.Ql \&:
following the name is not allowed.
String equates can't be exported or imported.
.Pp
.Sy Important note :
An
.Ic EQUS
can be expanded to a string that contains another
.Ic EQUS
and it will be expanded as well.
If this creates an infinite loop,
.Nm
will error out once a certain depth is
reached.
See the
.Fl r
command-line option in
.Xr rgbasm 1 .
Also, a macro can have inside an
.Ic EQUS
which references the same macro, which has the same problem.
.Pp
.It Ic MACRO
.Pp
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
.Sy IF
constructs.
.Pp
.Bd -literal -offset indent
MyMacro: MACRO
ld a,80
call MyFunc
ENDM
.Ed
.Pp
Note that a colon
.Ql \&:
following the macro's 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).
.Pp
The above example is a very simple macro.
You execute the macro by typing its name.
.Pp
.Bd -literal -offset indent
add a,b
ld sp,hl
MyMacro ;This will be expanded
sub a,87
.Ed
.Pp
When the assembler meets MyMacro it will insert the macro definition (the code enclosed in
.Ic MACRO
/
.Ic ENDM ) .
.Pp
Line continuations work as usual inside macros or lists of arguments of macros.
However, some characters need to be escaped, as in the following example:
.Pp
.Bd -literal -offset indent
PrintMacro : MACRO
PRINTT \[rs]1
ENDM
PrintMacro STRCAT("Hello"\[rs], \[rs]
" world\[rs]\[rs]n")
.Ed
.Pp
Suppose your macro contains a loop.
.Pp
.Bd -literal -offset indent
LoopyMacro: MACRO
xor a,a
\&.loop ld [hl+],a
dec c
jr nz,.loop
ENDM
.Ed
.Pp
This is fine, but only if you use the macro no more than once per scope.
To get around this problem there is a special string equate called
.Ic \[rs]@
that you will then expand to a unique string.
.Pp
.Ic \[rs]@
also works in REPT-blocks should you have any loops there.
.Bd -literal -offset indent
LoopyMacro: MACRO
xor a,a
\&.loop\[rs]@ ld [hl+],a
dec c
jr nz,.loop\[rs]@
ENDM
.Ed
.Pp
.Sy Important note :
Since a macro can call itself (or a different macro that calls the first one), there can be circular dependency problems.
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
.Sy EQUS
which references the same macro, which has the same problem.
.Pp
.Sy Macro Arguments
.Pp
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 then it loads the registers itself.
.Pp
And I can do that.
In macros you can get the arguments by using the special macro string equates
.Ic \[rs]1
through
.Ic \[rs]9 , \[rs]1
being the first argument specified on the calling of the macro.
.Pp
.Bd -literal -offset indent
LoopyMacro: MACRO
ld hl,\[rs]1
ld c,\[rs]2
xor a,a
\&.loop\[rs]@ ld [hl+],a
dec c
jr nz,.loop\[rs]@
ENDM
.Ed
.Pp
Now I can call the macro specifying two arguments, the first being the address and the second being a byte count.
The macro will then reset all bytes in this range.
.Pp
.Bd -literal -offset indent
LoopyMacro MyVars,54
.Ed
.Pp
Arguments are passed as string equates, although there's no need to enclose them in quotes.
Thus, an expression will not be evaluated first but passed directly.
This means that it's probably a very good idea to use brackets around
.Ic \[rs]1
to
.Ic \[rs]9
if you perform further calculations on them.
For instance, consider the following:
.Pp
.Bd -literal -offset indent
print_double: MACRO
PRINTV \1 * 2
ENDM
print_double 1 + 2
.Ed
.Pp
The
.Ic PRINTV
statement will expand to
.Ql PRINTV 1 + 2 * 2 ,
which will print 5 and not 6 as you might have expected.
.Pp
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
.Ic SHIFT .
.Pp
.Ic SHIFT
is a special command only available in macros.
Very useful in REPT-blocks.
It will shift the arguments by one to the left.
.Ic \[rs]1
will get the value of
.Ic \[rs]2 , \[rs]2
will get the value in
.Ic \[rs]3
and so forth.
.Pp
This is the only way of accessing the value of arguments from 10 to 256.
.Pp
.El
.Ss 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.
.Pp
Exporting of symbols has to be done manually, importing is done automatically if the assembler doesn't know where a symbol is defined.
.Bd -literal -offset indent
.Ic EXPORT Ar symbol1 Bq , Ar symbol2 , No ...
.Ed
.Pp
The assembler will make
.Ar symbol1 , symbol2
and so on accessible to other files during the link process.
.Bd -literal -offset indent
.Ic GLOBAL Ar symbol1 Bq , ...
.Ed
.Pp
If
.Ar symbol
is already defined, it will be exported, otherwise it will be imported.
Note that, since importing is done automatically, this keyword has the same effect as
.Ic EXPORT .
Note also that only exported symbols will appear in symbol and map files produced by
.Xr rgblink 1 .
.Ss Purging symbols
.Ic PURGE
allows you to completely remove a symbol from the symbol table as if it had never existed.
.Em USE WITH EXTREME CAUTION!!!
I can't stress this enough, you seriously need to know what you are doing.
DON'T purge a 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.
.Pp
.Bd -literal -offset indent
Kamikaze EQUS "I don't want to live anymore"
AOLer EQUS "Me too"
PURGE Kamikaze, AOLer
.Ed
.Pp
Note that string symbols that are part of a
.Ic PURGE
command
.Em will not be expanded
as the ONLY exception to the rule.
.Ss Predeclared Symbols
The following symbols are defined by the assembler:
.Pp
.Bl -column -offset indent "EQUS" "__ISO_8601_LOCAL__"
.It Sy Type Ta Sy Name Ta Sy Contents
.It Ic EQU Ta Ic @ Ta PC value
.It Ic EQU Ta Ic _PI Ta Fixed point \[*p]
.It Ic SET Ta Ic _RS Ta _RS Counter
.It Ic EQU Ta Ic _NARG Ta Number of arguments passed to macro
.It Ic EQU Ta Ic __LINE__ Ta The current line number
.It Ic EQUS Ta Ic __FILE__ Ta The current filename
.It Ic EQUS Ta Ic __DATE__ Ta Today's date
.It Ic EQUS Ta Ic __TIME__ Ta The current time
.It Ic EQUS Ta Ic __ISO_8601_LOCAL__ Ta ISO 8601 timestamp (local)
.It Ic EQUS Ta Ic __ISO_8601_UTC__ Ta ISO 8601 timestamp (UTC)
.It Ic EQU Ta Ic __UTC_YEAR__ Ta Today's year
.It Ic EQU Ta Ic __UTC_MONTH__ Ta Today's month number, 1-12
.It Ic EQU Ta Ic __UTC_DAY__ Ta Today's day of the month, 1-31
.It Ic EQU Ta Ic __UTC_HOUR__ Ta Current hour, 0-23
.It Ic EQU Ta Ic __UTC_MINUTE__ Ta Current minute, 0-59
.It Ic EQU Ta Ic __UTC_SECOND__ Ta Current second, 0-59
.It Ic EQU Ta Ic __RGBDS_MAJOR__ Ta Major version number of RGBDS.
.It Ic EQU Ta Ic __RGBDS_MINOR__ Ta Minor version number of RGBDS.
.It Ic EQU Ta Ic __RGBDS_PATCH__ Ta Patch version number of RGBDS.
.El
.Pp
.Sh DEFINING DATA
.Ss Declaring variables in a RAM section
.Ic DS
allocates a number of empty bytes.
This is the preferred method of allocating space in a RAM section.
You can also use
.Ic DB , DW
and
.Ic DL
without any arguments instead (see
.Sx Defining constant data
below).
.Pp
.Bd -literal -offset indent
DS 42 ; Allocates 42 bytes
.Ed
.Pp
Empty space in RAM sections will not be initialized.
In ROM sections, it will be filled with the value passed to the
.Fl p
command-line option, except when using overlays with
.Fl O .
.Ss Defining constant data
.Ic DB
defines a list of bytes that will be stored in the final image.
Ideal for tables and text.
Note that strings are not zero-terminated!
.Pp
.Bd -literal -offset indent
DB 1,2,3,4,"This is a string"
.Ed
.Pp
Alternatively, you can use
.Ic DW
to store a list of words (16-bit) or
.Ic DL
to store a list of double-words/longs (32-bit).
Strings are not allowed as arguments to
.Ic DW
and
.Ic DL .
.Pp
You can also use
.Ic DB , DW
and
.Ic DL
without arguments, or leaving empty elements at any point in the list.
This works exactly like
.Sy DS 1 ,
.Sy DS 2
and
.Sy DS 4
respectively.
Consequently,
.Ic DB ,
.Ic DW
and
.Ic DL
can be used in a
.Cm WRAM0
/
.Cm WRAMX
/
.Cm HRAM
/
.Cm VRAM
/
.Cm SRAM
section.
.Ss Including binary files
You probably have some graphics, level data, etc. you'd like to include.
Use
.Ic 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.
.Pp
.Bd -literal -offset indent
INCBIN "titlepic.bin"
INCBIN "sprites/hero.bin"
.Ed
.Pp
You can also include only part of a file with
.Ic INCBIN .
The example below includes 256 bytes from data.bin starting from byte 78.
.Pp
.Bd -literal -offset indent
INCBIN "data.bin",78,256
.Ed
.Ss Unions
Unions allow multiple memory allocations to share the same space in memory, like unions in C.
This allows you to easily reuse memory for different purposes, depending on the game's state.
.Pp
You create unions using the
.Ic UNION , NEXTU
and
.Ic ENDU
keywords.
.Ic NEXTU
lets you create a new block of allocations, and you may use it as many times within a union as necessary.
.Pp
.Bd -literal -offset indent
UNION
Name: ds 8
Nickname: ds 8
NEXTU
Health: dw
Something: ds 3
Lives: db
NEXTU
Temporary: ds 19
ENDU
.Ed
.Pp
This union will use up 19 bytes, as this is the size of the largest block
.Pq the last one, containing Sq Temporary .
Of course, as
.Sq Name ,
.Sq Health ,
and
.Sq Temporary
all point to the same memory
locations, writes to any one of these will affect values read from the others.
.Pp
Unions may be used in any section, but code and data may not be included.
.Sh THE MACRO LANGUAGE
.Pp
.Ss 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.
.Pp
.Bd -literal -offset indent
PRINTT "I'm the greatest programmer in the whole wide world\[rs]n"
PRINTI (2 + 3) / 5
PRINTV $FF00 + $F0
PRINTF MUL(3.14, 3987.0)
.Ed
.Pp
.Bl -inset
.It Ic PRINTT
prints out a string.
Be careful to add a line feed
.Pq Qq \[rs]n
at the end, as it is not added automatically.
.It Ic PRINTV
prints out an integer value in hexadecimal or, as in the example, the result of a calculation.
Unsurprisingly, you can also print out a constant symbol's value.
.It Ic PRINTI
prints out a signed integer value.
.It Ic PRINTF
prints out a fixed point value.
.El
.Ss Automatically repeating blocks of code
Suppose you want to unroll a time consuming loop without copy-pasting it.
.Ic REPT
is here for that purpose.
Everything between
.Ic REPT
and
.Ic ENDR
will be repeated a number of times just as if you had done a copy/paste operation yourself.
The following example will assemble
.Ql add a,c
four times:
.Pp
.Bd -literal -offset indent
REPT 4
add a,c
ENDR
.Ed
.Pp
You can also use
.Ic REPT
to generate tables on the fly:
.Pp
.Bd -literal -offset indent
; --
; -- Generate a 256 byte sine table with values between 0 and 128
; --
ANGLE = 0.0
REPT 256
db (MUL(64.0, SIN(ANGLE)) + 64.0) >> 16
ANGLE = ANGLE+256.0
ENDR
.Ed
.Pp
.Ic REPT
is also very useful in recursive macros and, as in macros, you can also use the special string symbol
.Ic \[rs]@ .
.Ic REPT
blocks can be nested.
.Ss Aborting the assembly process
.Ic FAIL
and
.Ic 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.
.Ic FAIL
and
.Ic WARN
take a string as the only argument and they will print this string out as a normal error with a line number.
.Pp
.Ic FAIL
stops assembling immediately while
.Ic WARN
shows the message but continues afterwards.
.Ss Including other source files
Use
.Ic 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
.Ic INCLUDE
calls infinitely (or until you run out of memory, whichever comes first).
.Pp
.Bd -literal -offset indent
INCLUDE "irq.inc"
.Ed
.Ss Conditional assembling
The four commands
.Ic IF , ELIF , ELSE ,
and
.Ic ENDC
are used to conditionally assemble parts of your file.
This is a powerful feature commonly used in macros.
.Pp
.Bd -literal -offset indent
IF NUM < 0
PRINTT "NUM < 0\[rs]n"
ELIF NUM == 0
PRINTT "NUM == 0\[rs]n"
ELSE
PRINTT "NUM > 0\[rs]n"
ENDC
.Ed
.Pp
The
.Ic ELIF
and
.Ic ELSE
blocks are optional.
.Ic IF
/
.Ic ELIF
/
.Ic ELSE
/
.Ic ENDC
blocks can be nested.
.Pp
Note that if an
.Ic ELSE
block is found before an
.Ic ELIF
block, the
.Ic ELIF
block will be ignored.
All
.Ic ELIF
blocks must go before the
.Ic ELSE
block.
Also, if there is more than one
.Ic ELSE
block, all of them but the first one are ignored.
.Ss Integer and boolean expressions
An expression can be composed of many things.
Expressions are always evaluated using signed 32-bit math.
.Pp
The most basic expression is just a single number.
.Pp
.Sy Numeric Formats
.Pp
There are a number of numeric formats.
.Pp
.Bl -column -offset indent "Fixed point (16.16)" "Prefix"
.It Sy Format type Ta Sy Prefix Ta Sy Accepted characters
.It Hexadecimal Ta $ Ta 0123456789ABCDEF
.It Decimal Ta none Ta 0123456789
.It Octal Ta & Ta 01234567
.It Binary Ta % Ta 01
.It Fixed point (16.16) Ta none Ta 01234.56789
.It Character constant Ta none Ta Qq ABYZ
.It Gameboy graphics Ta \` Ta 0123
.El
.Pp
The last one, Gameboy graphics, is quite interesting and useful.
The values are actually pixel values and it converts the
.Do chunky Dc data to Do planar Dc data as used in the Game Boy.
.Pp
.Bd -literal -offset indent
DW \`01012323
.Ed
.Pp
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.
.Pp
.Sy Operators
.Pp
A great number of operators you can use in expressions are available (listed from highest to lowest precedence):
.Pp
.Bl -column -offset indent "Operator"
.It Sy Operator Ta Sy Meaning
.It Li \&( \&) Ta Precedence override
.It Li FUNC() Ta Function call
.It Li ~ + - Ta Unary not/plus/minus
.It Li * / % Ta Multiply/divide/modulo
.It Li << >> Ta Shift left/right
.It Li & \&| ^ Ta Binary and/or/xor
.It Li + - Ta Add/subtract
.It Li != == <= Ta Boolean comparison
.It Li >= < > Ta Boolean comparison (Same precedence as the others)
.It Li && || Ta Boolean and/or
.It Li \&! Ta Unary Boolean not
.El
.Pp
The result of the boolean operators is zero when FALSE and non-zero when TRUE.
It is legal to use an integer as the condition for
.Sy IF
blocks.
You can use symbols instead of numbers in your expression if you wish.
.Pp
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 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 deferred to the linker).
In this case, the result is a constant that can be calculated at assembly time.
The instructions in the macro-language all require expressions that are constant.
.Pp
.Ss 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 expressions, 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 by 16 bits.
It follows that you can convert an integer to a fixed-point number by shifting it left.
.Pp
Some things are different for fixed-point math, though, which is why you have the following functions to use:
.EQ
delim $$
.EN
.Pp
.Bl -column -offset indent "ATAN2(x, y)"
.It Sy Name Ta Sy Operation
.It Fn DIV x y Ta $x \[di] y$
.It Fn MUL x y Ta $x \[mu] y$
.It Fn SIN x Ta $sin ( x )$
.It Fn COS x Ta $cos ( x )$
.It Fn TAN x Ta $tan ( x )$
.It Fn ASIN x Ta $asin ( x )$
.It Fn ACOS x Ta $acos ( x )$
.It Fn ATAN x Ta $atan ( x )$
.It Fn ATAN2 x y Ta Angle between $( x , y )$ and $( 1 , 0 )$
.El
.EQ
delim off
.EN
.Pp
These functions are useful for automatic generation of various tables.
Example: assuming a circle has 65536.0 degrees, and sine values are between
.Bq -1.0 ; 1.0 :
.Pp
.Bd -literal -offset indent
; --
; -- 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
ANGLE SET ANGLE+256.0
ENDR
.Ed
.Pp
.Ss String Expressions
The most basic string expression is any number of characters contained in double quotes
.Pq Ql \&"for instance" .
Like in C, the escape character is \[rs], and there are a number of commands you can use within a string:
.Pp
.Bl -column -offset indent "String"
.It Sy String Ta Sy Meaning
.It Li \[rs]\[rs] Ta Backslash
.It Li \[rs]" Ta Double quote
.It Li \[rs], Ta Comma
.It Li \[rs]{ Ta Curly bracket left
.It Li \[rs]} Ta Curly bracket right
.It Li \[rs]n Ta Newline ($0A)
.It Li \[rs]t Ta Tab ($09)
.It Li \[rs]1 - \[rs]9 Ta Macro argument (Only the body of a macros)
.It Li \[rs]@ Ta Label name suffix (Only in the body of macros and REPTs)
.El
.Pp
A funky feature is
.Ql {symbol}
within 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 with a dollar sign
.Sq $
prepended.
.Pp
It's possible to change the way numeric symbols are converted by specifying a print type like so:
.Ql {d:symbol} .
Valid print types are:
.Bl -column -offset indent "Print type" "Lowercase hexadecimal" "Example"
.It Sy Print type Ta Sy Format Ta Sy Example
.It Li d Ta Decimal Ta 42
.It Li x Ta Lowercase hexadecimal Ta 2a
.It Li X Ta Uppercase hexadecimal Ta 2A
.It Li b Ta Binary Ta 101010
.El
.Pp
Note that print types should only be used with numeric values, not strings.
.Pp
HINT: The
.Ic {symbol}
construct can also be used outside strings.
The symbol's value is again inserted directly.
.Pp
Whenever the macro-language expects a string you can actually use a string expression.
This consists of one or more of these functions (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!
.Pp
.Bl -column "STRSUB_str,_pos,_len"
.It Sy Name Ta Sy Operation
.It Fn STRLEN string Ta Returns the number of characters in Ar string .
.It Fn STRCAT str1 str2 Ta Appends Ar str2 No to Ar str1 .
.It Fn STRCMP str1 str2 Ta Returns negative if Ar str1 No is alphabetically lower than Ar str2 No , zero if they match, positive if Ar str1 No is greater than Ar str2 .
.It Fn STRIN str1 str2 Ta Returns the position of Ar str2 No in Ar str1 No or zero if it's not present Pq first character is position 1 .
.It Fn STRSUB str pos len Ta Returns a substring from Ar str No starting at Ar pos Po first character is position 1 Pc and with Ar len No characters.
.It Fn STRUPR str Ta Converts all characters in Ar str No to capitals and returns the new string.
.It Fn STRLWR str Ta Converts all characters in Ar str No to lower case and returns the new string.
.El
.Ss Character maps
When writing text that is meant to be displayed in the Game Boy, the ASCII characters used in the source code may not be the same ones used in the tileset used in the ROM.
For example, the tiles used for uppercase letters may be placed starting at tile index 128, which makes it difficult to add text strings to the ROM.
.Pp
Character maps allow the code to map strings up to 16 characters long to an abitrary 8-bit value:
.Pp
.Bd -literal -offset indent
CHARMAP "<LF>", 10
CHARMAP "í", 20
CHARMAP "A", 128
.Ed
.Pp
It is possible to create multiple character maps and then switch between them as desired.
This can be used to encode debug information in ASCII and use a different encoding for other purposes, for example.
Initially, there is one character map called
.Sy main
and it is automatically selected as the current character map from the beginning.
There is also a character map stack that can be used to save and restore which character map is currently active.
.Bl -column "NEWCHARMAP name, basename"
.It Sy Command Ta Sy Meaning
.It Ic NEWCHARMAP Ar name Ta Creates a new, empty character map called Ar name .
.It Ic NEWCHARMAP Ar name , basename Ta Creates a new character map called Ar name , No copied from character map Ar basename .
.It Ic SETCHARMAP Ar name Ta Switch to character map Ar name .
.It Ic PUSHC Ta Push the current character map onto the stack.
.It Ic POPC Ta Pop a character map off the stack and switch to it.
.El
.Pp
.Sy Note:
Character maps affect all strings in the file from the point in which they are defined, until switching to a different character map.
This means that any string that the code may want to print as debug information will also be affected by it.
.Pp
.Sy Note:
The output value of a mapping can be 0.
If this happens, the assembler will treat this as the end of the string and the rest of it will be trimmed.
.Pp
.Ss Other functions
There are a few other functions that do various useful things:
.Pp
.Bl -column "BANK(arg)"
.It Sy Name Ta Sy Operation
.It Fn BANK arg Ta Returns a bank number.
If
.Ar arg
is the symbol
.Ic @ ,
this function returns the bank of the current section.
If
.Ar arg
is a string, it returns the bank of the section that has that name.
If
.Ar arg
is a label, it returns the bank number the label is in.
For labels, as the linker has to resolve this, it can't be used when the expression has to be constant.
.It Fn DEF label Ta Returns TRUE if
.Ar label
has been defined.
.It Fn HIGH arg Ta Returns the top 8 bits of the operand if Ar arg No is a label or constant, or the top 8-bit register if it is a 16-bit register.
.It Fn LOW arg Ta Returns the bottom 8 bits of the operand if Ar arg No is a label or constant, or the bottom 8-bit register if it is a 16-bit register Pq Cm AF No isn't a valid register for this function .
.El
.Sh MISCELLANEOUS
.Ss Changing options while assembling
.Ic OPT
can be used to change some of the options during assembling from within the source, instead of defining them on the command-line.
.Pp
.Ic OPT
takes a comma-separated list of options as its argument:
.Pp
.Bd -literal -offset indent
PUSHO
OPT g.oOX ;Set the GB graphics constants to use these characters
DW `..ooOOXX
POPO
DW `00112233
.Ed
.Pp
The options that OPT can modify are currently:
.Cm b , e
and
.Cm g .
.Pp
.Ic POPO
and
.Ic PUSHO
provide the interface to the option stack.
.Ic PUSHO
will push the current set of options on the option stack.
.Ic 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 stack's number of entries is limited only by the amount of memory in your
machine.
.Sh SEE ALSO
.Xr rgbasm 1 ,
.Xr rgblink 1 ,
.Xr rgblink 5 ,
.Xr rgbds 5 ,
.Xr rgbds 7 ,
.Xr gbz80 7
.Sh HISTORY
.Nm
was originally written by Carsten S\(/orensen 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
.Lk https://github.com/rednex/rgbds .