Using machine code in BBC BASIC is best suited with allocating dynamic memory with DIM and then storing (assembling) the code to that area. If you don't assemble but have code ready, eg. loaded directly with PROC_lbytes, it is necessary that your machine code is relocatable, ie. contains no absolute address references (the machine code assumes that it is located at a certain ORG position in memory). You can relocatable code by omitting CALL and JP instructions and only use JR and DJNZ (jump relative) instruction. However, your program cannot be very large since relative jumps only range +/- 128 bytes in either direction from the instruction.

You can make truly relocatable machine code with the Mpm assembler, which is an integrated part of the Z88 Assembler Workbench. By using the '-R' option a small header is generated together with your code. The header contains a relocater routine and a relocation table. When your code is executed, it is automatically relocated to the current position in memory (just once). Subsequent calls to the code will just execute your code and not the relocater. With this option you can always store your machine code utilities with PROC_sbytes and at a later time allocate space with DIM for your code, and just load it into appropriate BBC BASIC memory. The code may be placed anywhere (in RAM).