---
title: "Undocumented Z-80 Instructions"
type: "article"
slug: "undocumented-z-80-instructions"
url: "http://localhost/article/undocumented-z-80-instructions/"
markdown_url: "http://localhost/article/undocumented-z-80-instructions.md"
published_at: "2026-03-13T23:55:29+00:00"
modified_at: "2026-03-13T23:55:30+00:00"
featured_image:
  url: "http://localhost/wp-content/uploads/2022/04/20230809-041332.jpg"
excerpt: "Explains that the Z80 processor supports undocumented 8-bit operations on the IX and IY index registers using the DD and FD prefix bytes. Any standard H or L register instruction preceded by DDh operates on the high/low bytes of IX (XH, XL), and FDh for IY (YH, YL), providing 4 additional 8-bit registers. Lists usable…"
category:
  - name: "Timelinez"
    slug: "timelinez"
    taxonomy: "category"
    url: "http://localhost/category/periodicals/timelinez/"
post_tag:
  - name: "Machine language programming"
    slug: "machine-language"
    taxonomy: "post_tag"
    url: "http://localhost/tag/machine-language/"
  - name: "TS 1000"
    slug: "ts1000"
    taxonomy: "post_tag"
    url: "http://localhost/tag/ts1000/"
  - name: "TS 2068"
    slug: "ts2068"
    taxonomy: "post_tag"
    url: "http://localhost/tag/ts2068/"
model:
  - name: "Timex/Sinclair 1000"
    slug: "ts-1000"
    taxonomy: "model"
    url: "http://localhost/model/ts-1000/"
  - name: "Timex/Sinclair 2068"
    slug: "ts-2068"
    taxonomy: "model"
    url: "http://localhost/model/ts-2068/"
indiv:
  - name: "Frank Moura"
    slug: "frank-moura"
    taxonomy: "indiv"
    url: "http://localhost/indiv/frank-moura/"
publication_r:
  id: 21571
  title: "Timelinez"
  type: "periodical"
  url: "http://localhost/periodical/timelinez/"
authors_r:
  - name: "Frank Moura"
    slug: "frank-moura"
    taxonomy: "indiv"
    url: "http://localhost/indiv/frank-moura/"
volume: "2"
issue: "6"
issues_articles:
  - id: 38803
    title: "Timelinez v2 n6"
    type: "issue"
    url: "http://localhost/issue/timelinez-v2-n6/"
pages: "5"
pubdate: "June 1984"
archive_link: false
---

Explains that the Z80 processor supports undocumented 8-bit operations on the IX and IY index registers using the DD and FD prefix bytes. Any standard H or L register instruction preceded by DDh operates on the high/low bytes of IX (XH, XL), and FDh for IY (YH, YL), providing 4 additional 8-bit registers. Lists usable instructions (LD, ADC, ADD, AND, CP, DEC, INC, OR, SBC, SUB) and notes BIT, IN, and OUT instructions do not work with these registers.