If your interest is swift turnaround, I'd definitely recommend not cross-assembly, but rather native builds on an emulator - for two reasons:
- There's no fixed addresses in QDOS except the hardware, everything is floating. This requires QL programs to be 100% position-independant code (there are exceptions, but that complicates things unnecessarily). Cross-development tools I know just aren't very good at producing such programs, they would assume a relocating loader which QDOS simply hasn't. Native tooling is aware of these restrictions and will warn you if broken or work around these.
- If there's one thing the QL was exceptionally good at at its time, it's its development tooling. You're coming from an Atari ST background, so you might know the GST Assembler and linker - Its ancestor is available on the QL and does a really nice job here.
You probably wouldn't want to use native editing tools, so you should work on an emulator that has direct access to its host file system - most current ones do.
How I work: I'm using MS Visual Studio Code as development environment, but run "make" on the emulated QL directly on the host FS. That saves me from tedious transfers of source code or binaries back and forth, and also allows modern Editors and CM systems like SVN or Git. The binary is directly available to be run in the emulator using native tooling like QMon (The best QL Monitor program).
Choice of emulators: Even if there are more emulators, the ones of choice really are QPC2, SMSQmulator and Q-Emulator. The former two are free, the latter is shareware. The former two are emulating "modern", SMSQ/E based "high-end"-QLs with lots of memory and better and faster CPUs, while the latter is more to the real thing, the closest you can get to in the QL world (don't expect it to be a 100% cycle-exact emulator like you might know from the ZX Spectrum scene, however). QPC expects a Windows environment (runs well on Wine or Parallels, however), while SMSqmulator is Java-based and doesn't expect an X86 system. Q-Emulator is available for both Windows and MacOS, its Windows version is free as long as you emulate a non-expanded QL (so ideal as a canary test environment).
Limits of emulation: There is no 100% emulator for the QL - all have their limits, Q-Emulator probably the closest to the real thing. It does, however have limits when emulating sound, and some peculiarities with the video hardware (it is, for example, the only emulator that supports the video page flipping the QL has, but on my systems with very noticeable delay.) Programming close to the silicon requires in fact real hardware.
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. So Im off to the tutorials sections and see if I can learn some way to poke data into screen address and display it!