That's not the case. Back in the day you would use some sort of "monitor" program to assemble that code (or to disassemble code already in memory). For example

That's not the case. Back in the day you would use some sort of "monitor" program to assemble that code (or to disassemble code already in memory). For example, the C64 Action Replay cartridge had a built-in monitor. Software machine code monitors and assemblers were also available.