View source for Create a plain ROM

A '''plain ROM''' is a [[Target|format]] of binary application to be write into a ROM.
The most common use case is the creation of '''game cartridges'''.
The build tool will generate a .ROM file that can be written to an EPROM (like with a Mega Flash ROM cartridge).

== Principles ==
A plain ROM, unlike a [[Create a mapped ROM|mapped ROM]], is constructed from a '''single block''' whose parts will always be seen by the Z80 at the same address.
Therefore, they cannot be more than '''64 KB''' in size (the maximum size visible to the Z80).
A program in ROM is detected by the MSX system thanks to a header located at addresses <tt>4000h</tt> or <tt>8000h</tt> but can in theory use any space between <tt>0000h</tt> and <tt>FFFFh</tt>.
Generally, a ROM uses one or more 16 KB pages (numbered from 0 to 3). Pages 1 and 2 can be used freely, but the system expects to have special content in pages 0 and 3:
* Page 0:
** The MSX BIOS with all standard functions.
** The interrupt handling code (ISR) at address <tt>0038h</tt>.
* Page 3:
** RAM with call stack and BIOS variables area.
** Secondary slot register at address <tt>FFFFh</tt>.

MSXgl was designed to easily '''override the limitations''' of page 0 by providing BIOS-independent functionality (all modules except... BIOS one) and by providing ISR code that can install automatically at <tt>0038h</tt>.
On the other hand, it is not easy to use page 3, especially because the '''call stack''' is located there.
However, it is possible to write code that will copy the contents of page 3 to VRAM (or to RAM in another page for MSX 2 or above).

== Targets ==
To create an plain ROM you just have to choice one of the following targets format:
{{:Target/Plain_ROM}}

For more detail, see [[Target]] article.

== Page data and code ==
By default, the [[Build tool]] will accumulate all your program's '''data and code''' from the ROM header address (<tt>4000h</tt>) in page 1. If the size of your data and code exceeds 16 KB, it will continue to accumulate them on the next page (page 2).

However, the [[Build tool]] also allows you to add data and code directly to any a given page of a plain ROM. 
This is especially useful for adding data to page 0 (before the ROM header), but also if you want to organize the code and data on pages 1 and 2 separately.

To do this, you just need to add at the root of your project, a file with a postfix <tt>_pX</tt> where <tt>X</tt> is the number of the page where to place this code (0 to 3).
And '''that's all'''! :)

The page file can be either in '''C''' (<tt>.c</tt>) or in '''assembler''' (<tt>.asm</tt>).
Data and code are accumulated from the following address:
* Page 0:
** <tt>0000h</tt> (for targets without ISR)
** <tt>0100h</tt> (for targets with ISR to prevent overwrite)
* Page 1: <tt>4000h</tt>
* Page 2: <tt>8000h</tt>
* Page 3: <tt>C000h</tt>

=== Exemple ===
Let's say you have a project named Monkey with the <tt>ROM_32K</tt> target format but for which you have almost no space left.
You can then switch to <tt>ROM_48K_ISR</tt> format in your project configuration (project_config.js) to have 16 KB more!
You don't have to change anything in your program (it still starts at <tt>4000h</tt>), but you can now add data or code to page 0 by including it in a <tt>monkey_p0.c</tt> file (you can also move there data or code from the main program).

Since you are using the <tt>ROM_48K_ISR</tt> format, the build tool will automatically add the necessary interrupt handling code for the system at address <tt>0038h</tt>.
As a result, all data and code contained in file <tt>monkey_p0.c</tt> will be added from address <tt>0100h</tt> (to avoid overwriting the ISR code).
This way, page 0 of your cartridge will always remain selected by the system and you won't have to juggle with slots or do inter-slot reading: The content of page 0 is directly accessible between addresses <tt>0100h</tt> and <tt>3FFFh</tt>.

All data labels or functions declared in <tt>monkey_p0.c</tt> can be used in your main program, but of course you have to provide their definitions (defined as <tt>extern</tt>).