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© 2024 Wantedly, Inc.
Porting mruby/c
for the SNES (Super Famicom)
RubyKaigi 2024
May 17 2024 - Ryota Egusa
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© 2024 Wantedly, Inc.
Ryota Egusa
@gedorinku
Software engineer
at Wantedly, Inc.
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[Ad]Wantedly, Inc. is a Platinum Sponsor
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Overview
● Running mruby/c on an actual SNES console
○ Developing SNES games using mruby/c
○ The mruby/c porting process
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The SNES
● Known as Super Famicom in Japan
● CPU: 65C816
○ 16 bit processor
○ 1.79 MHz, 2.68 MHz, 3.58 MHz
■ depending on the memory speed
○ Multiplication and division are handled either
by the coprocessor or implemented in
software
● RAM (W-RAM): 128KB
● VRAM: 64KB
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Why run mruby/c on SNES?
● Inspired by Yuji Yokoo's presentation at RubyKaigi
2022 about porting mruby/c to Sega Mega Drive
● I have been programming on the SNES as a hobby
before that
● Since 2023, the development of OSS C compiler for
65C816 has become more active (?)
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The hardware
● PPU (Picture Processing Unit) acts as a fixed
pipeline
● Writing values to PPU registers or VRAM causes
the PPU to output the display in sync with NTSC
(or PAL) signal timing
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BG and Sprite
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BG2
BG3
Sprite
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BG
● Tile Maps
○ Created by combining references to 8x8
images and color palettes
● The number of available BG (1 to 4) and the
number of colors per BG tile (4 to 256) vary
depending on the "BG Mode"
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Sprites
● You can set display positions and other settings
for each sprite
● Characters in games are generally rendered
using this feature
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Video and timing
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Vertical blanking interval
(VBlank)
● For NTSC:
● 262 scanlines /
frame
● Of those,
37 scanlines are
VBlank
Screen
Horizontal blanking interval
(HBlank)
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The Game implementation
while true
SNES::Pad.wait_for_scan
pad = SNES::Pad.current(0)
# (Game routine)
SNES.wait_for_vblank
end
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The Game implementation
while true
SNES::Pad.wait_for_scan
pad = SNES::Pad.current(0)
# (Game routine)
SNES.wait_for_vblank
end
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Wait for NTSC(or
PAL) Vertical
blanking interval
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The Game implementation
while true
SNES::Pad.wait_for_scan
pad = SNES::Pad.current(0)
# (Game routine)
SNES.wait_for_vblank
end
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The Game implementation
SNES::Bg.scroll(
1, camera_x, camera_y
)
SNES::OAM.set(
0, x, y, priority, 0, 0, frame, 0
)
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C compilers
● PVSnesLib
○ Includes the compiler, linker and wrappers for
the SNES I/O
● WDC Tools
○ The official tools by The Western Design
Center, Inc.
○ Includes the C compiler and linker
○ The source code is not publicly available
○ Does not support C99
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Address and C pointer
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$7e 8000
Bank address
(8 bit)
24 bit address
● CPU registers are 16 bit, address space is 24 bit
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Address and C pointer
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lda.w $8000
→Reads using the Data Bank Register (DB)
as the Bank Address.
lda.l $7e8000
→Reads from address $7e8000.
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Address and C pointer
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● Pointer Type:
○ 32 bit (only 24 bits are used)
● Global Variables:
○ All placed in the $7e bank and addressed with
16 bit addressing
● Function Calls:
○ All use 24-bit addressing (jsr.l/rtl)
○ The way addresses are pulled from the stack
changes between 16 bit and 24 bit on return
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mruby/c HAL Implementation
● Remove the implementation related to Scheduler
(rrt0c.c, rrt0.h)
● Only one function needs to be implemented.
● int hal_write(int fd, const void *buf, int
nbytes)
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mruby/c HAL Implementation
#define HAL_BUF_SIZE (1024)
static char hal_write_buf[HAL_BUF_SIZE];
int hal_write(
int fd, const void *buf,int nbytes
) {
// (Write to hal_write_buf)
}
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Debug
● There is no console available for outputting text
● Even attempting to display on the screen may fail
due to bugs
○ Use hal_write_buf for debugging output.
● Debugging is primarily done using an emulator
● Bugs that only reproduce on actual hardware can
be difficult to fix
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Mesen2 - emulator / debugger
●
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Debug
struct RObject { // mrbc_value
mrbc_vtype tt : 8;
union {
mrbc_int_t i;
...
struct RClass *cls;
struct RInstance *instance;
// Object#object_id
SET_INT_RETURN( v[0].i );
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Debug
● Problems difficult to reproduce in emulators:
○ Incorrect ROM formatting
○ Timing issues involving hardware
■ Example: Reading the Pad register
immediately after VBlank starts, which
should not be possible
● Solutions:
○ Use multiple emulators
○ Use the Programmable I/O pin
■ (I have never used this for debugging)
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Performance Improvement
● Scrolling just one BG layer results in about 8 fps
● Improved this to nearly 3 times faster
● Actions taken:
○ Utilizing enhancement chip
○ C compiler optimizations
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Enhancement chips
● Chips embedded within the cartridge
● Perform tasks such as graphics processing on
behalf of the console
● Examples
○ Super FX chip
■ For 2D and 3D graphics
○ ST018
■ ARMv3 32 bit processor
■ Used in “Hayazashi Nidan Morita Shogi 2”
for Shogi AI
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SA-1
● Uses the same 65C816 architecture
○ Not binary compatible, but porting is relatively
easy
● Additional memory (depends on the cartridge):
○ I-RAM: 2KB
○ BW-RAM: 128KB
● Differences from the S-CPU (CPU on SNES):
○ Cannot directly access registers such as the
PPU
○ Different memory mapping
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SA-1
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S-CPU
(65C816)
W-RAM
PPU
Game Cartridge
SA-1
(65C816)
I-RAM
BW-RAM
ROM
…
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SA-1 Memory mapping
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I-RAM
$00
$0000
$0800
$40
ROM
$8000
$10000
BW-RAM
$50
ROM
$60 $70
ROM
$80
I-RAM
$3000
$3800
Registers
$2000
I-RAM
ROM
I-RAM
Registers
$C0 $100
ROM
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SA-1 Memory mapping
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I-RAM
$00
$0000
$0800
$40
ROM
$8000
$10000
BW-RAM
$50
ROM
$60 $70
ROM
$80
I-RAM
$3000
$3800
Registers
$2000
I-RAM
ROM
I-RAM
Registers
$C0 $100
ROM
No mapping
for W-RAM
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SA-1 Memory mapping
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I-RAM
$00
$0000
$0800
$40
ROM
$8000
$10000
BW-RAM
$50
ROM
$60 $70
ROM
$80
I-RAM
$3000
$3800
Registers
$2000
I-RAM
ROM
I-RAM
Registers
$C0 $100
ROM
No registers
such as PPU
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SA-1 Memory mapping
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I-RAM
$00
$0000
$0800
$40
ROM
$8000
$10000
BW-RAM
$50
ROM
$60 $70
ROM
$80
I-RAM
$3000
$3800
Registers
$2000
I-RAM
ROM
I-RAM
Registers
$C0 $100
ROM
Twice as fast as BW-RAM.
Used for the stack.
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SA-1 Memory mapping
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I-RAM
$00
$0000
$0800
$40
ROM
$8000
$10000
BW-RAM
$50
ROM
$60 $70
ROM
$80
I-RAM
$3000
$3800
Registers
$2000
I-RAM
ROM
I-RAM
Registers
$C0 $100
ROM
Mapped to the same
location in the S-CPU.
Convenient for memory
sharing.
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SA-1
● Describes metadata about the cartridge,
such as the size of the ROM
● $FFD6 $35
○ $30: SA-1
○ $05: ROM + coprocessor + RAM + battery
● $FFD8 $07
○ RAM size
○ 1<<7 = 128KB
ROM Header
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SA-1
void call_s_cpu(void (*target_func)(),
size_t args_size, ...);
call_s_cpu(bg_set_scroll, sizeof(int) * 3,
1, x, y);
Calling the S-CPU
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Writes to shared memory.
S-CPU simply polls this memory.
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SA-1 Calling the S-CPU
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$0000
$2000
$3000
$3800
args of
target_func
call_s_cpu_targe
t_func’s frame
Copy and call
target_func
SA-1 stack
mapped in
S-CPU
S-CPU stack
args of
target_func
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Running mruby/c on SA-1
● S-CPU and SA-1 operate in parallel
● When SNES is reset, S-CPU executes the address
of the Reset vector
○ At this point, SA-1 is not yet active.
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Running mruby/c on SA-1
lda #__start_sa1 ; Set Reset vector
sta $2203
sep #$20 ; Set A register to 8 bit
stz $2200 ; Run SA-1
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Running mruby/c on SA-1
__start_sa1:
(Initialize memory and registers here)
jsr.l sa1_main
int sa1_main(void) {
(Run mruby/c VM)
}
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Demo
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Feature work
● Performance Improvement
○ Further optimize the C compiler
○ Optimize memory usage (use I-RAM as much
as possible)
○ Support DMA using Array (like object) in
mruby/c
● Allow to run without SA-1
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Conclusion
● There's still a lot of potential to improve
performance and stability of C compiler for
65C816
● To run mruby/c on SNES, you need the
enhancement chip for now
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https://github.com/gedorinku/snes-ruby
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References
● https://github.com/mrubyc/mrubyc
● https://rubykaigi.org/2022/presentations/yujiyokoo.html
● https://github.com/alekmaul/pvsneslib
● https://github.com/alekmaul/tcc
● https://github.com/SourMesen/Mesen2
● https://github.com/VitorVilela7/SMW-SA1-Pack
● SNESdev Wiki
○ https://snes.nesdev.org/wiki/SNESdev_Wiki
● SFC Development Wiki
○ https://wiki.superfamicom.org/
● W65C816S 8⁄16–bit Microprocessor
○ https://www.westerndesigncenter.com/wdc/documentati
on/w65c816s.pdf
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