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Hello everyone,

I'm in the process of getting a simple homebrew game going for game gear. I did use the tutorials on here to get started, so thank you very much for those!

Now I'm looking at how I can code in a save game function. Does anyone know if there is any documentation on this? Please let me know where to find this information. I have spent a few hours googling around, but haven't made much progress.

Secondly, does anyone know of any current homebrew games that do have a save game function?

Thank you for any help.

I've used it a few times. You write to the register at $fffc to map the save RAM into the memory space, then just read and write to it from $8000 onwards. It's good practice to write a marker in there to allow you to detect when it's initialised/present.

http://www.smspower.org/Development/Mappers#RAMMapping

I think it is used in http://www.smspower.org/Homebrew/Picross-SMS and http://www.smspower.org/Homebrew/Ono-SMS .

Hi,

I'm working on saving user inputs to the battery-backed external ram, in order to record player sprite movements, so that I can 'play back' these movements during attract screens. This is the core recorder loop:

run_recorder:
;
call await_frame_interrupt
call get_input_ports
;
; Save the input ports at every 255th frame (4-5 sec).
ld a,(frame_counter)
dec a
ld (frame_counter),a
or a
jp nz,+
SELECT_EXTRAM
ld hl,extram_header ; Get extram header address.
call get_word
ld a,(InputPorts) ; Read the variable set by GetInputPorts.
ld (hl),a ; Write the InputPort state to extram.
ld hl,extram_header ; Increment the header (word).
call inc_word
SELECT_ROM
+:
jp main_loop

The key to this save feature lies in switching between external ram and rom. I use these two macros:

; =============================================================================
; M A C R O S
; =============================================================================
; -----------------------------------------------------------------------------
.macro SELECT_EXTRAM
; -----------------------------------------------------------------------------
ld a,(BANK_CONTROL)
or SET_EXTRAM_BIT
ld (BANK_CONTROL),a
.endm
; -----------------------------------------------------------------------------
.macro SELECT_ROM
; -----------------------------------------------------------------------------
ld a,(BANK_CONTROL)
and RESET_EXTRAM_BIT
ld (BANK_CONTROL),a
.endm

With these definitions:

.equ SET_EXTRAM_BIT %00001000
.equ RESET_EXTRAM_BIT %11110111

.equ EXTRAM_START $8000
.equ EXTRAM_SIZE $4000

.equ BANK_CONTROL $fffc

Of course this is not an example on saving a game, but it shows how to select the external ram; once external ram is selected, write savegame stuff to it (level, points, lives, etc.) - emulators will create a file on the disk to emulate the contents of this battery-backed, external ram. You can load from this ram the same way. Select it, then read from it as required, and select the rom again when done (all this stuff happens in slot 2).

#Edit: Argh, Maxim beat me to it :)

Thank you guys. I'll read up on this!

Hang on
Thank you for the code example. Can I ask what RAM chip are you using for hardware or are you doing this all with emulation?

You are welcome. Regarding ram chip, I don't know. I test my code on emulators (Emulicious and Meka) and on real hardware through an Everdrive (the above code is - for the moment - only tested on emulators).

Author	Message
doublehacker Joined: 21 Feb 2017 Posts: 11	Programming Homebrew with save game fuction Posted: Mon Mar 20, 2017 8:54 pm
doublehacker Joined: 21 Feb 2017 Posts: 11	Hello everyone, I'm in the process of getting a simple homebrew game going for game gear. I did use the tutorials on here to get started, so thank you very much for those! Now I'm looking at how I can code in a save game function. Does anyone know if there is any documentation on this? Please let me know where to find this information. I have spent a few hours googling around, but haven't made much progress. Secondly, does anyone know of any current homebrew games that do have a save game function? Thank you for any help.

Maxim Site Admin Joined: 19 Oct 1999 Posts: 14726 Location: London	Posted: Mon Mar 20, 2017 9:14 pm Last edited by Maxim on Mon Mar 20, 2017 9:15 pm; edited 1 time in total
	I've used it a few times. You write to the register at $fffc to map the save RAM into the memory space, then just read and write to it from $8000 onwards. It's good practice to write a marker in there to allow you to detect when it's initialised/present. http://www.smspower.org/Development/Mappers#RAMMapping I think it is used in http://www.smspower.org/Homebrew/Picross-SMS and http://www.smspower.org/Homebrew/Ono-SMS .

hang-on Joined: 23 Mar 2013 Posts: 611 Location: Copenhagen, Denmark	Posted: Mon Mar 20, 2017 9:14 pm
	Hi, I'm working on saving user inputs to the battery-backed external ram, in order to record player sprite movements, so that I can 'play back' these movements during attract screens. This is the core recorder loop: run_recorder: ; call await_frame_interrupt call get_input_ports ; ; Save the input ports at every 255th frame (4-5 sec). ld a,(frame_counter) dec a ld (frame_counter),a or a jp nz,+ SELECT_EXTRAM ld hl,extram_header ; Get extram header address. call get_word ld a,(InputPorts) ; Read the variable set by GetInputPorts. ld (hl),a ; Write the InputPort state to extram. ld hl,extram_header ; Increment the header (word). call inc_word SELECT_ROM +: jp main_loop The key to this save feature lies in switching between external ram and rom. I use these two macros: ; ============================================================================= ; M A C R O S ; ============================================================================= ; ----------------------------------------------------------------------------- .macro SELECT_EXTRAM ; ----------------------------------------------------------------------------- ld a,(BANK_CONTROL) or SET_EXTRAM_BIT ld (BANK_CONTROL),a .endm ; ----------------------------------------------------------------------------- .macro SELECT_ROM ; ----------------------------------------------------------------------------- ld a,(BANK_CONTROL) and RESET_EXTRAM_BIT ld (BANK_CONTROL),a .endm With these definitions: .equ SET_EXTRAM_BIT %00001000 .equ RESET_EXTRAM_BIT %11110111 .equ EXTRAM_START $8000 .equ EXTRAM_SIZE $4000 .equ BANK_CONTROL $fffc Of course this is not an example on saving a game, but it shows how to select the external ram; once external ram is selected, write savegame stuff to it (level, points, lives, etc.) - emulators will create a file on the disk to emulate the contents of this battery-backed, external ram. You can load from this ram the same way. Select it, then read from it as required, and select the rom again when done (all this stuff happens in slot 2). #Edit: Argh, Maxim beat me to it :)

doublehacker Joined: 21 Feb 2017 Posts: 11	Posted: Tue Mar 21, 2017 4:00 am
doublehacker Joined: 21 Feb 2017 Posts: 11	Thank you guys. I'll read up on this! Hang on Thank you for the code example. Can I ask what RAM chip are you using for hardware or are you doing this all with emulation?

hang-on Joined: 23 Mar 2013 Posts: 611 Location: Copenhagen, Denmark	Posted: Tue Mar 21, 2017 7:13 am
	You are welcome. Regarding ram chip, I don't know. I test my code on emulators (Emulicious and Meka) and on real hardware through an Everdrive (the above code is - for the moment - only tested on emulators).

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