Get the sourcecode

There is currently no binary package of VBCC available. This will change when a new official release is made, or when it is working well enough for a beta release from me.

The sources are in several parts:

It is also a good idea to download and read the manuals for each of them.



make CPU=unsp SYNTAX=std

Copy vasmunsp_std and vobjdump somewhere in your PATH


Copy vlink executable in your PATH


make TARGET=unsp

The first time this asks you various questions about the host system. If you're not on a strange platform, the default answers should be correct.

Copy vc and vbccunsp executables from the bin/ directory in your PATH

Target configuration

The vc executable is a compiler frontend. It does not itself compile the C code, but it knows how to call the compiler, assembler and linker to perform the usual steps of a compilation.

However, this knowledge is not hardcoded in the tool, instead, it is loaded from a configuration file. This way, a single version of the compiler frontend can be used to work with many different CPU architectures and platforms.

Create a vbcc directory somewhere. When building anything with VBCC, you need to set the VBCC environment variable to point to that directory:

export VBCC=/path/to/vbccdir

The content of this directory:

  • config/
    • vsmile
  • targets/unsp-vsmile/
    • lib/
      • startup.o
    • vlink.cmd

The configuration file

This file located in config/vsmile is the entry point for the configuration. It is enabled by passing the +vsmile option to the vc compiler frontend.

-cc=vbccunsp -I"$VBCC"/targets/unsp-vsmile/include -quiet %s -o= %s %s -O=%ld
-ccv=vbccunsp -I"$VBCC"/targets/unsp-vsmile/include %s -o= %s %s -O=%ld
-as=vasmunsp_std -quiet -Fvobj %s -o %s
-rm=rm %s
-rmv=rm %s
-ld=vlink -ole -b rawbin1 -Cvbcc -T"$VBCC"/targets/unsp-vsmile/vlink.cmd -L"$VBCC"/targets/unsp-vmsile/lib "$VBCC"/targets/unsp-vsmile/lib/startup.o %s %s -o %s
-ldv=vlink -ole -b rawbin1 -Cvbcc -T"$VBCC"/targets/unsp-vsmile/vlink.cmd -L"$VBCC"/targets/unsp-vmsile/lib "$VBCC"/targets/unsp-vsmile/lib/startup.o %s %s -o %s -Mmapfile
-l2=vlink -ole -b rawbin1 -Cvbcc -T"$VBCC"/targets/unsp-vsmile/vlink.cmd -L"$VBCC"/targets/unsp-vmsile/lib %s %s -o %s
-l2v=vlink -ole -b rawbin1 -Cvbcc -T"$VBCC"/targets/unsp-vsmile/vlink.cmd -L"$VBCC"/targets/unsp-vmsile/lib %s %s -o %s -Mmapfile

This defines the commands to run for each step of the compilation: cc for compiling, as for assembling, and ld for linking. The variants with a v suffix are for verbose output.

Some of the paths used refer to the VBCC environment variable we have set before.

For the details of each option used, refer to the documentation of the corresponding tool, but in short:

  • vbccunsp is called with the sourcefile and optimization options,
  • vasmunsp_std is called to assemble the resulting file and output a vobj relocatable object file,
  • vlink is used to link all the vobj files together, it outputs the final cartridge image using the rawbin1 output module (raw binary), with the -ole option to generate it in little endian (as expected by all existing tools).

The frontend looks at the file extension of the input and output, as well as some of the options (like -c, to compile and generate a .o file, but not link), and automatically determines which tools to call.

The linker script

The file vlink.cmd contains the linker script. This tells vlink about the memory layout and what to do with the code and data.

        ram : org = 0x0000, len = 0x2800
        lorom : org = 0x0000, len = 0xfff4
        res : org = 0xfff5, len = 11
        rom : org = 0x10000, len = 0x3f0000
        .bss (NOLOAD): { *(.bss) } > ram
        .empty: { RESERVE(0x4000); } > lorom
        .rodatal: { *(.rodata) } > lorom
        .rodata: { *(.rodata) *(.rodata2) } > rom
        .textl: { *(.text) } > lorom
        .text: { *(.text) } > rom
        .ctorsl: { *(.ctors) } > lorom
        .ctors: { *(.ctors) } > rom
        .dtorsl: { *(.dtors) } > lorom
        .dtors: { *(.dtors) } > rom
        .data: { *(.data) } > ram AT > lorom
        .res: { *(.res); } > res
        __BS = ADDR(.bss);
        __BL = SIZEOF(.bss);
        __DS = ADDR(.data);
        __DL = SIZEOF(.data);
        __STACK = 0x2800;

First of all this script defines several memory regions:

  • RAM from 0000 to 2800 (all addresses are of course in 16-bit words)
  • “low” ROM starting at 0000 and until the reset vectors
  • Reset vectors section
  • “high” ROM after the reset vectors until the end of the cartridge

Note that the “low ROM” section starts at 0 and overlaps the RAM. This is unusual, but it is how V.Smile cartridges are done: they have 4000 words at the start which are unused and unreachable by the CPU.

The script then defines sections. These follow the usual conventions of C compilers:

  • bss is for uninitialized data, and is stored at the start of the RAM. Typically, the tilemap will be allocated there, as well as global and static variables if they are not explicitly initialized in the code.
  • The empty section is just for these empty 4000 words at the start of the cartridge. It is put in the “low ROM” area.
  • All following sections are duplicated, with one instance loaded into low ROM and the other into high ROM. This tells the linker to put things as much as possible in the low ROM area, and then, if there is no space there anymore, start using the high ROM area.
  • rodata is for constants (such as strings),
  • text is for the executable code,
  • ctors and dtors is for code that should be called before and after the main() function (not implemented yet)
  • data is for initialized variables. This one is a bit special, since variables have to be in RAM, but their initial values will be stored in ROM. A bit of code in the startup (see below) is used to initialize the variables before calling the main() function.
  • res is for the reset and interrupt vectors. These are loaded at address FFF5 in the matching section. We will see below in the startup file how they are filled in.

Finally, the script defines a few variables that will be usable from assembler code, to indicate the start and size of the bss section and of the data section as well as the stack pointer initial value. These are also used by the startup file.

And the last line defines the vector table as the “entry point”. This allows the linker to follow references from this entry point to other parts of the code (functions, variables, …) and determine what is actually used. Functions that are not called from anywhere can be removed at this stage, for example.

FIXME this linker script needs some changes to allow developers to more easily put some things explicitly in the “low ROM” section. For example, reset vectors have to be there. Also check how alignment works for the things that need it.

The startup file

; TODO move this font out of here into the projects that use it.
.section .rodata
.incbin font.bin
.section .text
; Default handler for all interrupts (except RESET).
; Just return from the interupt without doing anything.
; Defined as weak symbols, so, if an application redefines them, the version from the app will
; be used instead.
; IRQ3:
; IRQ5:
; Handler for the RESET vector.
; Does the early initialization, then jumps into the main function.
        ; Disable interrupts since we're probably not ready to handle them yet
        IRQ OFF
        ; Set up the stack at the end of RAM
        LD SP, 0x27FF
        ; Copy the initialized variables into RAM
        ; __DL = length of variables section
        ; __DS = start of variables section in ROM
        LD R1, 0x0000
        LD R2, __DS
        LD R3, __DL
        JZ gomain
        ADD R3, R1
        LD R4, (R2++)
        ST R4, (R1++)
        CMP R3, R1
        JNE _startloop
        ; Finally, jump into the main function
        GOTO main
; The interrupt vectors section, contains pointers to the interrupt handlers
.section .res,"adr"
.globl _vectortable
.size _vectortable, 11
.2byte BREAK
.2byte FIQ
.2byte _start
.2byte IRQ0
.2byte IRQ1
.2byte IRQ2
.2byte IRQ3
.2byte IRQ4
.2byte IRQ5
.2byte IRQ6
.2byte IRQ7

This file defines the initialization routine and a few other things.

The first thing it currently does is including the font binary file used for rendering text.

  • FIXME move that elsewhere, it has nothing to do in the startup file, but it was convenient for me to put it there for now.

Then come default definitions for most of the interrupt vectors. They all execute a RETI instruction without doing anything. Currently, the ones used by the application need to be commented out here. Later, this should be replaced with weak symbols that can be replaced if the application provides something else.

The _start function is the reset vector, and so it is the first code that will run when the CPU resets. It turns off the interrupts, initializes the stack pointer, and copies the initial values for variables in the data section into RAM. Then, it jumps into the main function.

Finally, there is the table of reset vectors.

Of course, vbcc will expect this in vobj (.o) format, so let's assemble it:

vasmunsp_std -ile -Fvobj -o startup.o crt0.asm

and place the resulting file startup.o in the target/unsp-vsmile/lib/ directory, where our frontend config file says to look for it.

Note: the -ile option tells vasm that files included using the incbin directive are in little endian. This gives the same results as the official unSP toolchain when importing binary files.

Running the compiler

To compile a .c file into a .o file:

vc +vsmile -c file.c -o

To generate a binary file from .o files:

vc +vsmile -ole -o cartridge.bin *.o

And of course you can run the resulting cartridge in MAME if you want:

mame -debug -debugger qt -rompath /path/to/vsmile/bios/ vsmile -cart ./cartridge.bin -nomax

Generating debug symbols for MAME

Use the -v option to vbcc to enable verbose mode, this will also generate a mapfile containing info about the generated binary.

The mapfile list of symbols can be turned into a file that the MAME debugger can use to create comments:

sed mapfile -ne 's!  0x\(.*\) \(.*\):.*!comadd \1,\2!p' > symbols.mame

Then in MAME, use “source symbols.mame” to load it. You can switch the disassembly view to show comments, and it will be very helpful to understand where you are when stepping through the code. Even local labels are exported, which can be matched with .ic2 files to compare the intermediate representation with the generated code. Very useful when debugging the compiler.

TODO list


  • There is an internal compiler error when compiling the sound demo
  • long int values are mostly untested and broken in various places (code is missing, they will be handled the same as 1-byte values)
  • Contiki crashes when trying to call a function pointer
  • Probably more problems waiting in Contiki once that is fixed
  • Try to compile larger projects such as Copyright Infringement to find more bugs

Memory addressing

  • Make it possible to put things in low ROM explicitly using dedicated sections (needed for example for the reset vectors)
  • Add a “far pointer” type. The current pointer type is only 16 bit so it can only point into the current data segment
  • Add a “far pointer” type also for functions

Startup and library

  • The startup script should clear the bss segment to 0
  • Implement division and modulo for “long” types, signed vs unsigned, etc.
  • Floating point support routines
  • Move font.bin outside of crt0/startup, and provide a more convenient way to handle resources (some Makefile rules maybe?)


  • Due to the allocation of temporary registers by the backend and related register spilling, this sequence can appear in the generated assembly code:
POP  R1, R1, [SP]
PUSH R1, R1, [SP]
LD R1, ...

The POP and PUSH can be eliminated, they are useless. This should be done using the peephole optimizer since the register spilling is generated by the backend and does not come from ICs.

  • ALLOCREG ICs are not always located just before the first use of the register. This results in registers being marked as allocated for the next few ICs, when in fact it's not. Propagating the ALLOCREG down until the first IC that actually uses the register would allow more use of temporary registers.
  • Optimize TEST ICs if the latest generated instruction already set the flags to the correct value. In particular, sequences like:
LD R1, ...
CMP R1, 0
  • The frontend does not make use of special addressing modes (post-increment, etc). The backend should detect sequences of ICs that can be collapsed to a single instruction. The IC sequence is something like:
Move pointer to register
Add 1 to register
Store to pointer-register

(there are a few variants for pre and post increments, and some other ICs may be inserted in between, making this not so easy to track).

  • When setting several registers to the same value, there is one IC to move a constant to each of them. This results in a lot of repeated LD Rx,nn ; ST Rx,[yyyy] with the same nn. This can be optimized at the IC level (create an allocreg to store the constant) or maybe a bit lower (just check if the previous IC used the same register with the same value?)
  • BRA IC is always translated as GOTO, use LJMP instead so short jumps are used where possible
vbcc.txt · Last modified: 2024/06/01 14:09 by pulkomandy
CC Attribution 4.0 International
Driven by DokuWiki Recent changes RSS feed Valid CSS Valid XHTML 1.0