The crt0 - C startup routine - shall be configured accoring to ROM image size and/or ROM type.

To configure ROM image size and/or ROM type for your application, define CONFIG_ROM_TYPE variable in your Makefile as follows:

 makefile
# (in your project's Makefile)
# Select ROM type.
# - 16k     : to make 16KiB rom image (0x4000-0x7fff)
# - 32k     : to make 32KiB rom image (0x4000-0xbfff)
# - ascii16 : to make MegaROM image (ROM type: ASCII16)
# - ascii8  : to make MegaROM image (ROM type: ASCII8)
CONFIG_ROM_TYPE = 16k

If you want to use BDOS (MSX-DOS / Disk BASIC) functionality in your application, in addition to the above, set CONFIG_CRT0_MOD_INIT to init_x or init_r.

 makefile
# (in your project's Makefile)
CONFIG_CRT0_MOD_INIT = init_x

Make-variables for crt0 configuration

CONFIG_ROM_TYPE

Select ROM type of the ROM image to be built.

CONFIG_ROM_TYPE ?= 16k in default.

value	explanation
`16k`	16KiB plain ROM image, starting at 0x4000.
`32k`	32KiB plain ROM image, starting at 0x4000.
`ascii8`	MegaROM image with ASCII8 rom mapper support, starting at 0x4000.
`ascii16`	MegaROM image with ASCII16 rom mapper support, starting at 0x4000.

CONFIG_CRT0_MOD_ROM_HEADER

value	explanation
`app_rom_header`	ROM header for plain ROM image of application.
`app_megarom_header`	ROM header for MegaROM image of application.

The value is automatically selected according to CONFIG_ROM_TYPE.

CONFIG_CRT0_MOD_INIT

Select init module of the ROM image to be built.

CONFIG_CRT0_MOD_INIT ?= init_0 in default.

init_0 starts main() immediately on initialization phase of the ROM cartridge. No other system extension is initialized.

In contrast to init_0, init_x or init_r starts main() after initialization phase of all system extension is done. This means that:

all system extension is ready to use. if it exists,
but maybe, some RAM space and startup time are consumed.

The table below shows the runtime environment at application startup:

value	CPU mode	BDOS	other system extension
`init_0`	Z80 mode	N/A	N/A
`init_x`	Z80 mode	MSX-DOS 1 / Disk BASIC 1.x (1)	available if exists
`init_r`	R800 DRAM mode (2)	MSX-DOS 2 / Disk BASIC 2.x (1) (2)	available if exists

(1): Available only if a FDC exists.
If Shift key was pressed during startup time, BDOS is not available even if a Floppy Disk Controller (FDC) exists.

(2): MSXturboR or later.
On MSX, MSX2, or MSX2+ the behaviour is same as init_x.
If 1 key was pressed during startup time, the behaviour will be same as init_x even if on MSXturboR.

CONFIG_CRT0_MOD_START

value	explanation
`start_16k`	startup routine for plain 16KiB ROM image.
`start_32k`	startup routine for plain 32KiB ROM image.
`start_megarom`	startup routine for MegaROM image.

The value is automatically selected according to CONFIG_ROM_TYPE.

CONFIG_CRT0_MOD_ROM_MAPPER

value	explanation
`rom_ascii8`	use ASCII8 ROM mapper
`rom_ascii16`	use ASCII16 ROM mapper

The value is automatically selected according to CONFIG_ROM_TYPE.

CONFIG_CRT0_MOD_LIBS

value	explanation
`get_slot`	get the current slot of page #n.

The value is automatically selected according to CONFIG_ROM_TYPE and CONFIG_CRT0_MOD_INIT.

USE_ALL_EXTENSIONS (deprecated)

USE_ALL_EXTENSIONS ?= in default.

USE_ALL_EXTENSIONS = 1 implies CONFIG_CRT0_MOD_INIT = init_x for backward compatibility.

Pre-defined crt0 configurations

Some pre-defined crt0 configurations are in ${LIBMSX_HOME}/mk/ folder.

Instead of defining CONFIG_ROM_TYPE, you can include one of the following into the Makefile.

${LIBMSX_HOME}/mk/16k.4000.mk
${LIBMSX_HOME}/mk/32k.4000.mk
${LIBMSX_HOME}/mk/ascii8.mk
${LIBMSX_HOME}/mk/ascii16.mk

The below table shows that which component is integrated into crt0, for each pre-defined configuration:

	16k.4000.mk	32k.4000.mk	ascii8.mk	ascii16.mk
`CONFIG_CRT0_MOD_ROM_HEADER`	`app_rom_header`	`app_rom_header`	`app_megarom_header`	`app_megarom_header`
`CONFIG_CRT0_MOD_INIT`	(3)	(3)	(3)	(3)
`CONFIG_CRT0_MOD_START`	`start_16k`	`start_32k`	`start_megarom`	`start_megarom`
`CONFIG_CRT0_MOD_ROM_MAPPER`			`rom_ascii8`	`rom_ascii16`
`CONFIG_CRT0_MOD_LIBS`	`get_slot`	`get_slot`	`get_slot`	`get_slot`

(3): Choose one of the init module; init_0, init_x, or init_r. init_0 is selected in default.

${LIBMSX_HOME}/mk/16k.4000.mk

 makefile
CONFIG_ROM_TYPE = 16k
CONFIG_CRT0_MOD_INIT ?= init_0

${LIBMSX_HOME}/mk/32k.4000.mk

 makefile
CONFIG_ROM_TYPE = 32k
CONFIG_CRT0_MOD_INIT ?= init_0

${LIBMSX_HOME}/mk/ascii8.mk

 makefile
CONFIG_ROM_TYPE = ascii8
CONFIG_CRT0_MOD_INIT ?= init_0

${LIBMSX_HOME}/mk/ascii16.mk

 makefile
CONFIG_ROM_TYPE = ascii16
CONFIG_CRT0_MOD_INIT ?= init_0