Ruby developer and internet japester Aaron Patterson has [1]published a REPL for 64-bit x86 assembly language, enabling interactive coding in the lowest-level language of all.

REPL stands for "read-evaluate-print loop", and REPLs were first seen in Lisp development environments such as Lisp Machines. They allow incremental development: programmers can write code on the fly, entering expressions or blocks of code, having them evaluated – executed – immediately, and the results printed out. This was viable because of the way Lisp blurred the lines between interpreted and compiled languages; these days, they're a standard feature of most scripting languages.

Patterson has previously offered ground-breaking developer productivity enhancements such as an analogue [2]terminal bell and performance-enhancing [3]firmware for the Stack Overflow [4]keyboard . This only has Ctrl, C, and V keys for extra-easy copy-pasting, but Patterson's firmware removes the tedious need to hold control.

[5]It's 2021 and someone's written a new Windows 3.x mouse driver. Why now?

[6]Assembly language, arcade games, and YouTube: The Reg speaks to former Microsoft engineer Dave Plummer

[7]HP's solution to running GPU-accelerated Linux apps on high-end Z workstations: Rely on Microsoft's WSL2

[8]IBM Java CTO: Devs shouldn't have to learn Docker, K8s, 30 other things to deploy an app

We suspect that the Assembler REPL came out of Patterson's work on his own just-in-time compiler for Ruby, [9]TenderJIT . He does such [10]things for his day job. And like TenderJIT, the primary purpose of AsmREPL is educational.

Patterson explained: "I wrote it because I can never remember what the test instruction does to the zero flag. Every time I use the instruction I have to look up the docs. Looking up docs is fine, but running code in a REPL helps me remember things better."

[11]

AsmREPL works by assembling code into a shared memory buffer using Patterson's own [12]Fisk , an x86-64 assembler written in Ruby, then using the Unix ptrace system call to monitor what the code is doing. Windows users will have to use [13]WSL .

[14]

It's not quite the same as a machine-code monitor, as loved by coding pioneers in the early 8-bit days, but it looks like a fun tool if you're learning to write device drivers or other very low-level code. ®

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[1] https://github.com/tenderlove/asmrepl

[2] https://analogterminalbell.com/

[3] https://github.com/tenderlove/qmk_firmware

[4] https://stackoverflow.blog/2021/09/28/become-a-better-coder-with-this-one-weird-click/

[5] https://www.theregister.com/2021/11/29/a_new_windows_3x_mouse/

[6] https://www.theregister.com/2021/01/06/plummer_interview/

[7] https://www.theregister.com/2021/11/12/hp_z_wsl2/

[8] https://www.theregister.com/2018/02/22/ibm_java_cto_john_duimovich_interview/

[9] https://github.com/tenderlove/tenderjit

[10] https://github.com/shopify/yjit

[11] https://pubads.g.doubleclick.net/gampad/jump?co=1&iu=/6978/reg_software/applications&sz=300x50%7C300x100%7C300x250%7C300x251%7C300x252%7C300x600%7C300x601&tile=2&c=2YaatGQNi5@J4IaZ74tbJYAAAAMw&t=ct%3Dns%26unitnum%3D2%26raptor%3Dcondor%26pos%3Dtop%26test%3D0

[12] https://github.com/tenderlove/fisk

[13] https://www.theregister.com/2021/11/17/ubuntu_team_systemd_windows_wsl2/

[14] https://pubads.g.doubleclick.net/gampad/jump?co=1&iu=/6978/reg_software/applications&sz=300x50%7C300x100%7C300x250%7C300x251%7C300x252%7C300x600%7C300x601&tile=4&c=44YaatGQNi5@J4IaZ74tbJYAAAAMw&t=ct%3Dns%26unitnum%3D4%26raptor%3Dfalcon%26pos%3Dmid%26test%3D0

[15] https://whitepapers.theregister.com/