--- title: ZX81 KEYBOARD Subroutine—Commentary | daily.dev description: A detailed technical analysis of the ZX81 keyboard scanning subroutine from the ROM at address 0x02BB. The routine uses an 8-iteration loop to scan an 8×5... canonical: https://daily.dev/posts/zx81-keyboard-subroutine-commentary-fr2z6olcl twitter:card: summary_large_image twitter:site: @dailydotdev og:type: website og:site_name: daily.dev og:title: ZX81 KEYBOARD Subroutine—Commentary | daily.dev og:description: A detailed technical analysis of the ZX81 keyboard scanning subroutine from the ROM at address 0x02BB. The routine uses an 8-iteration loop to scan an 8×5... og:url: https://daily.dev/posts/zx81-keyboard-subroutine-commentary-fr2z6olcl og:image: https://api.daily.dev/og/posts/Fr2z6OLcl.png og:image:alt: ZX81 KEYBOARD Subroutine—Commentary og:image:width: 1200 og:image:height: 630 og:locale: en --- > ## Documentation Index > Fetch the complete documentation index at: https://daily.dev/llms.txt > Use this file to discover all available pages before exploring further. # ZX81 KEYBOARD Subroutine—Commentary **[Two Stop Bits](https://daily.dev/sources/twostopbits)** · 10 min read · 0 upvotes · 0 comments ## Summary A detailed technical analysis of the ZX81 keyboard scanning subroutine from the ROM at address 0x02BB. The routine uses an 8-iteration loop to scan an 8×5 keyboard matrix, encoding keypresses into HL registers where bits correspond to horizontal and vertical zones. The commentary walks through the Z80 assembly code instruction-by-instruction, explaining how the routine handles the shift key specially, uses bit manipulation tricks like SBC A,A for conditional logic, and interfaces with the keyboard hardware through I/O ports. Includes keyboard zone diagrams, hardware schematics, a Z80 assembly cheatsheet, and a BASIC program to inspect the scan codes. ## Full article daily.dev links to this article rather than hosting it. Read it at the original source: ## Similar posts on daily.dev - [Sinclair 4K BASIC for the ZX80](https://daily.dev/posts/sinclair-4k-basic-for-the-zx80-icvlkapuu) · Two Stop Bits · 0 upvotes · 0 comments - [For Such A Small Program, ZX81 1K Chess Sure Packs A Lot In](https://daily.dev/posts/for-such-a-small-program-zx81-1k-chess-sure-packs-a-lot-in-1nixokoj7) · Hackaday · 0 upvotes · 0 comments - [Back To Basics: Hacking On Key Matrixes](https://daily.dev/posts/back-to-basics-hacking-on-key-matrixes-1az55yoey) · Hackaday · 0 upvotes · 0 comments --- Tags: [#assembly](https://daily.dev/tags/assembly), [#embedded](https://daily.dev/tags/embedded), [#hardware](https://daily.dev/tags/hardware) [View this post on daily.dev](https://daily.dev/posts/zx81-keyboard-subroutine-commentary-fr2z6olcl) ```json {"@context":"https://schema.org","@graph":[{"@type":"Organization","@id":"https://daily.dev/#organization","name":"daily.dev","url":"https://daily.dev","logo":{"@type":"ImageObject","url":"https://daily.dev/apple-touch-icon.png","width":180,"height":180},"sameAs":["https://twitter.com/dailydotdev","https://github.com/dailydotdev","https://www.linkedin.com/company/daily-dev-ltd"]},{"@type":"WebSite","@id":"https://daily.dev/#website","url":"https://daily.dev","name":"daily.dev","publisher":{"@id":"https://daily.dev/#organization"},"potentialAction":{"@type":"SearchAction","target":{"@type":"EntryPoint","urlTemplate":"https://daily.dev/search?q={search_term_string}"},"query-input":"required name=search_term_string"}}]} {"@context":"https://schema.org","@type":"TechArticle","headline":"ZX81 KEYBOARD Subroutine—Commentary","url":"https://daily.dev/posts/zx81-keyboard-subroutine-commentary-fr2z6olcl","mainEntityOfPage":{"@type":"WebPage","@id":"https://daily.dev/posts/zx81-keyboard-subroutine-commentary-fr2z6olcl"},"datePublished":"2026-02-17T06:08:00.017Z","dateModified":"2026-03-15T07:34:43.081Z","description":"A detailed technical analysis of the ZX81 keyboard scanning subroutine from the ROM at address 0x02BB. 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