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Open ROMs development guide

NOTE: this document is a preliminary version, definitely incomplete!

In-depth protocol description

Guidelines for software developers for Open ROMs compatibility

Most important: always test your software on the original ROMs too. If your code works on the Open ROMs, but fails on the original firmware - than something is clearly wrong, and it might stop working once we make Open ROMs more compatible for one reason or another.

Unofficial ROM APIs

Please, do not use any internal/unofficial Open ROMs API. They WILL change in an incompatible way, you may believe me. Using unofficial API of the original ROMs is OK if the routine is marked as supported in the implementation status document.

Compatibility with Open ROMs for MEGA65

The MEGA65 Open ROMs build aims to provide a C64-compatible memory map in range $00000-$0FFFF. For $10000 and above, everything is still subject to change.

Please note, that certain ROM routines are moved to additional MEGA65 ROM area (2x8KB is not enough to implement full ROM functionality and additional features) - original locations contain just short compatibility code which remaps memory, calls the real routine, and restores default mapping (that means all blocks unmapped). This shouldn't be a problem for original C64 software, but can make a difference for MEGA65-aware code; thus, please follow these simple rules:

  • do not call ROM routines with .B set to anything other than 0 (and .B set to 0x40 or above is a receipe for disaster!)
  • be aware that ROM routines may restore the default memory mapping by means of MAP/EOM

Open ROMs interrupt handlers do not touch the ROM mapping, range 0x4000-0xBFFF can be freely mapped without disabling IRQ or NMI. The .Z register is mostly safe to use too - some routines might use it, but they will always restore it's previous state. Please be aware, that for maximum compatibility our IRQ/NMI handlers DO NOT store/restore .Z register.

API extensions

PETSCII - C128

When C128 keyboard support is compiled-in, Open ROMs provides support for a few PETSCII codes:

PETSCII code description
$07 bell
$1B ESC key
$84 HELP key

There is no PETSCII code for TAB key - we can't utilize the ones from C128 or C65 because it conflicts with some C64 codes.

Bucky keys

SHFLAG ($028D) variable is extended to support additional bucky keys on C128 and C65 keyboards:

bits 7-6 bit 5 bit 4 bit 3 bit 2 bit 1 bit 0
reserved NO SCRL CAPS LOCK ALT CTRL VENDOR SHIFT

Bits 3 and 4 are extension to the original variable, compatible with the C128 ROMs API. There is no official way to retrieve NO_SCRL status as of yet. The 40/80 key status cannot be retrieved in C64 mode at all (C128 hardware limitation).

NMI vector modification

Before modifying NMI vector, set it's high byte to 0. The ROM routine considers such address invalid and won't try to call the custom code, thus preventing a crash if interrupt happens in the worst possible moment.

MEGA65 native mode

MEGA65 native mode imlementation has started. Important changes so far:

  • Uses VIC-IV features to implement 80-column screen mode; the $0400-$07FF area is utilized by ROM to provide additional features.
  • Slightly different chargen. On C64 machines pressing C= + G or C= + M key combinations produces the same visual result as C= + H or C= + N, respectively - despite keycap printing suggests they should produce tinner bars. This was intentional change in the C64 ROMs (compared to VIC-20), to prevent colour problems on 80s era TVs. Screen font used in the legacy mode duplicates this change, but the font intended for native mode has this change reverted back. Additionally, the π sign is available for lower/upper case set too.

Tinkering with Open ROMs

Make sure to read the main README first - although any help with the project is always welcome, you need to be aware of the possible legal problems we want to avoid.

Make targets

List of the most important make targets:

target description
all builds all ROMs, places them in 'build' subdirectory
clean removes all the compilation results and intermediate files
updatebin upates ROMs in 'bin' subdirectory - with embedded version string, for release
testsimilarity launches the similarity tool, see README
test builds the 'custom' configuration, launches it using VICE emulator
test_generic builds the default ROMs, for generic C64/C128, launches using VICE
test_generic_x128 as above, but launches C128 emulator instead
test_generic_crt builds the default ROM with extended cartridge image, launches using VICE
test_mega65 builds the MEGA65 ROM, launches it using XEMU emulator
test_ultimate64 builds the Ultimate 64 configuration, launches it using VICE
test_ultimate64_crt as above, ROM with extended cartriodge image
test_hybrid builds a hybrid ROM (Open ROMs Kernal + original BASIC), launches it using VICE
test_testing builds a rather odd testing configuration, launches it using VICE

Code segments and ROM layouts

Currently there are 4 ROM layouts defined:

  • STD - with 2 code segments: BASIC ($A000-$E4D2, where $C000-$DFFF is a skip-gap for RAM and I/O area) and KERNAL ($E4D3 - $FFFF)
  • CRT - where BASIC becames BASIC_0 and KERNAL becames KERNAL_0, with additional BASIC/KERNAL segments are placed on the required cartridge image
  • M65 - where BASIC becames BASIC_0 and KERNAL becames KERNAL_0, and several additional segments are added for extended functionality
  • X16 - memory layout for the Commander X16 machine

Expect at least one more, U64, if the planned additional ROM banks support for Ultimate 64 gets implemented. To check for current ROM layout or code segment use KickAssembler preprocessor defines, like ROM_LAYOUT_STD, SEGMENT_M65_BASIC_0, SEGMENT_X16_KERNAL_1.

Fixed location vs floating routines

There are two types of routines (in this chapter routine = single source file):

  • fixed location - they are always placed in the location specified by developer; if it's not possible, an error is produced during the compilation
  • floating - their location in the ROM segment is determined during the compilation by our build segment tool; from the developer point of view it is random

File name of the fixed location routine adheres to the scheme: addr.name.s, where addr is a 4-digit hexadecimal number. They should be accompanied with *.interop file, describing why the location got fixed - see README for more information.

Handling different ROM layouts

For handling different ROM layouts, our build tool provides support for pragma-like comments, in the form:

;; #LAYOUT# <rom-layout> <code-segment> <action> <parameters>

where:

  • separators are mandatory - at least one space or tabulation has to be placed
  • <rom-layout> - currently STD or M65, asterisk symbol can be used to match any layout
  • <code-segment> - KERNAL, BASIC, KERNAL_0, KERNAL_1, etc., asterisk symbol can be used to match any segment
  • <action> - what to do when layout and segment match:
action description
#IGNORE ignore the file completely
#TAKE compile the file normally
#TAKE-FLOAT compile the file, but force it to be floating
#TAKE-HIGH as above, but always place routine at $E000 or above
#TAKE-OFFSET shifts the fixed-location address by hex offset given as parameter

The first match counts, all the remaining ones are dropped. Examples:

;; #LAYOUT# STD *        #TAKE
;; #LAYOUT# *   KERNAL_0 #TAKE
;; #LAYOUT# *   *        #IGNORE

This is a very common one. For standard ROM layout just take the file. For non-standard ones take it only for compiling KERNAL_0 segment, ignore it for anything other (like KERNAL_1). Another example:

;; #LAYOUT# M65 KERNAL_0 #TAKE-FLOAT
;; #LAYOUT# M65 KERNAL_1 #TAKE
;; #LAYOUT# *   *        #IGNORE

This can be found in a private (internal) vector table of KERNAL_1 segment of M65 layout. For standard ROM part (KERNAL_0 segment) take it as floating routine (for KERNAL_0 segment the file provides only labels for the vector table). For KERNAL_1 segment of M65 layout, take it as a fixed location-routine; in such case the file will provide a vector table itself. For memory layout other than M65, just ignore the file. Yet another example:

;; #LAYOUT# M65 KERNAL_0 #TAKE
;; #LAYOUT# *   *        #IGNORE

This is used for memory mapping helper routines, which are only needed for MEGA65 build. They will be placed in KERNAL_0 segment in such case. And the last example:

;; #LAYOUT# STD *        #TAKE
;; #LAYOUT# M65 KERNAL_0 #TAKE
;; #LAYOUT# M65 KERNAL_1 #TAKE-FLOAT
;; #LAYOUT# *   *        #IGNORE

This is for the routine, that on MEGA65 goes to KERNAL_1 segment, but can still be transparently called using fixed location in KERNAL_0 segment. The code might, for example, look this way:

ROUTINE_NAME:

!ifdef SEGMENT_M65_KERNAL_0 {

    jsr map_KERNAL_1
    jsr (VK1__ROUTINE_NAME)    // execute routine from KERNAL_1 segment
    jmp map_NORMAL

} else {

    // real routine, ended with RTS
}

Last, but not least - the #TAKE-HIGH is intended to be used for BASIC routines, which should still be available after the main BASIC ROM is banked out.

MEGA65 computer-based DOS

Both the internal DOS and all the IEC operations leave with CPU speed set to 1 MHz (in case of legacy compatibility mode) or 40 MHz (in case of native mode). Also the fast interrupts and badline emulation is set according to the mode.

TODO

CPU-specific optimizations

TODO

Labels and VICE debugging

Make targets which launch the VICE emulator pass al the label to the built-in monitor. Problem can arise if we have multiple labels pointing the same address - in such case VICE monitor displays a warning, and just one of the labels is being recognized. To avoid this problem please select a single main label for the given memory location, and make sure all the others contain __ - such labels will be filtered out from the symbol file for VICE.