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Improve linker scripts a little (#1275)
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* Allow for optional sections in linker scripts
  These are more useful for frameworks/toolchains.

* Check for an active mem region everywhere
  Do you like segfaults? Too bad!

* Allow the address to be floating in linker scripts
  Try and make the life of SDCC interop easier.

* Also validate alignment when floating

* Overhaul the linker script manual page
  Documenting the new features, but also restructuring the
  existing documentation to make the manual page (hopefully)
  easier to understand.
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@ISSOtm
ISSOtm authored Dec 25, 2023
1 parent 7b199d7 commit ccf9dcb
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202 changes: 129 additions & 73 deletions man/rgblink.5
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.Nm rgblink
.Nd linker script file format
.Sh DESCRIPTION
The linker script is an external file that allows the user to specify the order of sections at link time and in a centralized manner.
The linker script is a file that allows specifying attributes for sections at link time, and in a centralized manner.
There can only be one linker script per invocation of
.Nm ,
but it can be split into several files
.Pq using the Ic INCLUDE No directive .
.Ss Basic syntax
The linker script syntax is line-based.
Each line may have a directive or section name, a comment, both, or neither.
Whitespace (space and tab characters) is used to separate syntax elements, but is otherwise ignored.
.Pp
A linker script consists of a series of bank declarations, each optionally followed by a list of section names (in double quotes) or directives.
All reserved keywords (bank types and directive names) are case-insensitive; all section names are case-sensitive.
.Pp
Any line can contain a comment starting with
Comments begin with a semicolon
.Ql \&;
that ends at the end of the line.
.Bd -literal -offset indent
; This line is a comment
ROMX $F ; start a bank
"Some functions" ; a section name
ALIGN 8 ; a directive
"Some array"

WRAMX 2 ; start another bank
org $d123 ; another directive
"Some variables"
.Ed
.Pp
Numbers can be in decimal or hexadecimal format
.Pq the prefix is Ql $ .
It is an error if any section name or directive is found before setting a bank.
character, until the end of the line.
They are simply ignored.
.Pp
Files can be included by using the
.Ic INCLUDE
keyword, followed by a string with the path of the file that has to be included.
Keywords are composed of letters and digits (but they can't start with a digit); they are all case-insensitive.
.Pp
The possible bank types are:
.Cm ROM0 , ROMX , VRAM , SRAM , WRAM0 , WRAMX , OAM
and
.Cm HRAM .
Unless there is a single bank, which can occur with types
.Cm ROMX , VRAM , SRAM
and
.Cm WRAMX ,
it is mandatory to specify a bank number after the type.
.Pp
Section names in double quotes support the same character escape sequences as strings in
Numbers can be written in decimal format, or in binary using the
.Ql %
prefix, or in hexadecimal using the
.Ql $
prefix (hexadecimal digits are case-insensitive).
Note that unlike
.Xr rgbasm 5 ,
specifically
an octal
.Ql &
prefix is not supported, nor are
.Ql _
digit separators.
.Pp
Strings begin with a double quote, and end at the next (non-escaped) double quote.
Strings must not contain literal newline characters.
Most of the same character escapes as
.Xr rgbasm 5
are supported, specifically
.Ql \e\e ,
.Ql \e" ,
.Ql \en ,
Expand All @@ -56,51 +50,113 @@ and
.Ql \et .
Other backslash escape sequences in
.Xr rgbasm 5
are only relevant to assembly code and do not apply in section names.
are only relevant to assembly code and do not apply in linker scripts.
.Ss Directives
.Bl -tag -width Ds
.It Including other files
.Ql Ic INCLUDE Ar path
acts as if the contents of the file at
.Ar path
were copy-pasted in place of the
.Ic INCLUDE
directive.
.Ar path
must be a string.
.It Specifying the active bank
.Tg region
The active bank can be set by specifying its type (memory region) and number.
The possible types are:
.Ic ROM0 , ROMX , VRAM , SRAM , WRAM0 , WRAMX , OAM ,
and
.Ic HRAM .
The bank number can be omitted from the types that only contain a single bank, which are:
.Ic ROM0 ,
.Ic ROMX No if Fl t No is passed to Xr rgblink 1 ,
.Ic VRAM No if Fl d No is passed to Xr rgblink 1 ,
.Ic WRAM0 ,
.Ic WRAMX No if Fl w No is passed to Xr rgblink 1 ,
.Ic OAM ,
and
.Ic HRAM .
.Pq Ic SRAM No is the only type that can never have its bank number omitted.
.Pp
When a new bank statement is found, sections found after it will be placed right from the beginning of that bank.
If the linker script switches to a different bank and then comes back to a previous one, it will continue from the last address that was used.
After a bank specification, the
.Dq current address
is set to the last value it had for that bank.
If the bank has never been active thus far, the
.Dq current address
defaults to the beginning of the bank
.Pq e.g. Ad $4000 No for Ic ROMX No sections .
.It Changing the current address
A bank must be active for any of these directives to be used.
.Pp
The only three directives are
.Ic ORG ,
.Ic ALIGN ,
and
.Ic DS :
.Bl -bullet
.It
.Ic ORG Ar addr
sets the address in which new sections will be placed to
.Ql Ic ORG Ar addr
sets the
.Dq current address
to
.Ar addr .
It can not be lower than the current address.
.It
.Ic ALIGN Ar addr
or
.Ic ALIGN Ar addr , Ar offset
will increase the address until it is aligned to the specified boundary
.Po it tries to set to
This directive cannot be used to move the address backwards:
.Ar addr
must be greater than or equal to the
.Dq current address .
.Pp
.Ql Ic FLOATING
causes all sections between it and the next
.Ic ORG
or bank specification to be placed at addresses automatically determined by
.Nm .
.Pp
.Ql Ic ALIGN Ar addr , Ar offset
increases the
.Dq current address
until it is aligned to the specified boundary (i.e. the
.Ar align
lowest bits of the address are equal to
.Ar offset ) .
If
.Ar offset
the number of bits specified by
.Ar align :
for example,
is omitted, it is implied to be 0.
For example, if the
.Dq current address
is $0007,
.Ql ALIGN 8
will align to $100 ,
and
would set it to $0100, and
.Ql ALIGN 8 , 10
will align to $10A
.Pc .
.It
.Ic DS
will increase the address by the specified non-negative amount.
would set it to $000A.
.Pp
.Ql Ic DS Ar size
increases the
.Dq current address
by
.Ar size .
The gap is not allocated, so smaller floating sections can later be placed there.
.El
.Ss Section placement
A section can be placed simply by naming it (with a string).
Its bank is set to the active bank, and its address to the
.Dq current address .
Any constraints the section already possesses (whether from earlier in the linker script, or from the object files being linked) must be consistent with what the linker script specifies: the section's type must match, the section's bank number (if set) must match the active bank, etc.
In particular, if the section has an alignment constraint, the address at which it is placed by the linker script must obey that constraint; otherwise, an error will occur.
.Pp
.Sy Note :
The bank, alignment, address and type of sections can be specified both in the source code and in the linker script.
For a section to be able to be placed with the linker script, the bank, address and alignment must be left unassigned in the source code or be compatible with what is specified in the linker script.
For example,
.Ql ALIGN[8]
in the source code is compatible with
.Ql ORG $F00
in the linker script.
After a section is placed, the
.Dq current address
is increased by the section's size.
This must not increase it past the end of the active memory region.
.Pp
The section must have been defined in the object files being linked, unless the section name is followed by the keyword
.Ic OPTIONAL .
.Sh EXAMPLES
.Bd -literal -offset indent
; This line contains only a comment
ROMX $F ; start a bank
"Some functions" ; a section name
ALIGN 8 ; a directive
"Some \e"array\e""

WRAMX 2 ; start another bank
org $d123 ; another directive
"Some variables"
.Ed
.Sh SEE ALSO
.Xr rgbasm 1 ,
.Xr rgbasm 5 ,
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