If you plan to write any machine code routines of significant size, the only logical method is to use an assembler.
This assembler comes in a cartridge with an A5 size instructions booklet. The cartridge auto-runs on switch on and resides in the block starting at $9000. As a consequence you have 30,719 bytes free.
This is, however, sufficient to write enough source code for about 3-4K of object code at one sitting. If your source code is very large, you can split the source code into several files and these can be assembled in a linked sequence.
The actual assembler uses standard mnemonics for the source code. The Basic editor is used to create the source code and normal saving and loading is used to handle files.
The assembly process uses three passes and consequently, allows you to use labels and incorporate simple arithmetic in the source code. The code is assembled to RAM rather than to tape or disk: a surprisingly handy system.
The usual pseudo-op codes (WOR, END, BYT and TXT) are available, as are commands for listing the assembled code and generating a label table.