Seeking Inspiration from Jupiter Ace Forth

March 23, 2020

Brad Nelson / @flagxor

Jupiter Ace

  • Company: Jupiter Cantab
  • Founders: Richard Altwasser (hardware) and Steven Vickers (software)
  • Had worked on ZX-81, ZX-Spectrum at Sinclair
  • Named after Automatic Computing Engine (early British Computer)

Specifications

  • CPU: Zilog Z80A 3.25MHz
  • ROM: 8KB
  • RAM: 1KB base (expandable to 49KB)
  • Video RAM: 1KB charmap, 1KB charset (write only)
  • 10 x 4 = 40 QWERTY Membrane keyboard
  • Character set: ASCII-1967 + ↑, £ and ©

Jupiter Ace Forth

  • 134 Built-in Words
  • Double and Float Words
  • Special DEFINER and COMPILER Words
  • No disk drive → Dictionary Editing

Sinclair ZX-80


  • Z80 3.25 MHz
  • 1KB RAM!
  • 4KB ROM BASIC

Sinclair ZX-81

  • Z80 3.25 MHz
  • 1KB RAM!
  • 8KB ROM BASIC

Sinclair ZX Spectrum

  • Z80A 3.25MHz
  • 16-colors
  • 16KB ROM BASIC, 16-128KB RAM

Jupiter Ace Graphics

Zilog Z80 CPU

  • 8-bit CPU, with 16-bit Register pairing
  • Binary compatiable with Intel 8080
  • RST instructions similar to INT on x86

8080 Registers

  • A, Flags (AF)
  • B, C (BC)
  • D, E (DE)
  • H, L (HL)
  • SP (16-bit)
  • PC (16-bit)

Z80 Registers

  • A, Flags (AF)
  • B, C (BC)
  • D, E (DE)
  • H, L (HL)
  • IX (16-bit)
  • IY (16-bit)
  • SP (16-bit)
  • PC (16-bit)

Forth Register Use

  • AF - Temporary
  • BC - Temporary
  • DE - Temporary
  • HL - Temporary
  • IX - System Variables ($3C00)
  • IY - Interpreter Pointer
  • SP - Return Stack Pointer (IP on top)
  • $3C3B - Data Stack Pointer
  • RST n - Used for core stack ops

Interpreter (fast)

; DE = *IP++;  (IP==*SP)
POP     HL
LD      E,(HL)
INC     HL
LD      D,(HL)
INC     HL
PUSH    HL
; HL = *DE++;
EX      DE,HL
LD      E,(HL)
INC     HL
LD      D,(HL)
INC     HL
EX      DE,HL
; goto HL
JP      (HL)

Interpreter (slow)

  • Check for stack underflow
  • Check free memory
  • Check BREAK key
  • Call iy_fast

Memory Map

  • 0000 - 1FFF = ROM
  • 2000 - 23FF = 768 charmap + 256 PAD (CPU wins)
  • 2400 - 27FF = 768 charmap + 256 PAD (Video wins)
  • 2800 - 2BFF = 1KB charset (CPU wins)
  • 2C00 - 2FFF = 1KB charset (Video wins)
  • 3000 - 3BFF = RAM 1KB x 3 mirrors
  • 3C00 - 3FFF = RAM 1KB
  • 3C00 - FFFF = Expansion RAM (0-48KB)

Defining Words

  • Explicit syntax for defining words
  • DOES> only allowed in DEFINER
  • Allows redefinition, updating existing created words (caveats)

Defining Words

DEFINER CONSTANT
  ,
DOES>
  @ ;

Compiler Words

  • Explicit syntax for words with inline data
  • RUNS> only allowed in COMPILER
  • Implicitly IMMEDIATE
  • Fixed or variable (-1) length
  • Allows redefinition, updating existing references (caveats)
  • Inline data isn't visible

Compiler Words

2 COMPILER LITERAL
  ,
RUNS>
  @ ;

Compiler Words

-1 COMPILER S"
  ASCII " WORD DUP 1+ SWAP C@
  DUP , 0
  DO
    DUP C@ C, 1+
  LOOP
  DROP
RUNS>
  DUP 2+ SWAP @ ;

Compiler Words

List

Edit

REDEFINE

  • Replace a word with the last word defined
  • Allows for editing prior definitions
  • Allows for renaming
  • Moves words in memory! - Subtle ramifications

Built-In Dictionary Format

  • NAME (n bytes)
  • Link Field (2 bytes)
  • Name Length & Flags (1 bytes, 0x40=immediate)
  • Code Field (2 bytes)

User Dictionary Format

  • NAME (n bytes)
  • Length Field (2 bytes, length - name)
  • Link Field (2 bytes)
  • Name Length & Flags (1 bytes, 0x40=immediate)
  • Code Field (2 bytes)

Relocatable Code

  • Only some word forms and dictionary structure is fixed up
  • Words are assumed to be position independent
  • CREATEd and VARIABLEs should not store dictionary addresses
  • Dispatch tables done with regular words

Dispatch Table

: mux ( n -- xt ) 2* r> + @ ;
: operations ( n -- xt ) mux + - * / ;
7 3 1 operations execute .
==
4 OK

Vocabularies

  • Structured as a tree
  • Linked list of all vocabularies
  • Tricks to hook up child vocabularies

Vocabularies

  • VOCABULARY name
  • DEFINITIONS
  • CURRENT (lookup)
  • CONTEXT (writing)

Vocabulary Structure

  • NAME (n bytes)
  • Link Field (2 bytes)
  • Name Length & Flags (1 bytes, 0x40=immediate)
  • Code Field (2 bytes)
  • Newest Word (2 bytes)
  • Zero (1 byte, fake name length field)
  • Next Vocabulary (2 bytes)

Vocabularies

Hello

Vocabularies

Hello

Vocabularies

Hello

Vocabularies

Hello

Word Types

  • Listable
    • Regular words
    • DEFINER words, COMPILER words
    • COMPILER word references (minus data)
    • Comments/Print String
  • Not Listable
    • Code words, Defined words
    • CREATEed words, VARIABLEs, CONSTANTs
    • VOCABULARY words

Save and Load

  • Cassette tape storage
  • Optional filename
  • Saves whole RAM dictionary
  • SAVE, LOAD, VERIFY
  • BSAVE, BLOAD, BVERIFY

Emulators

  • xAce
  • Jupiler (Retrolandia)
  • SpudACE, no$zx, EightyOne, Ace32xp, Vace, MESS, Multi Machine

Inspirations

  • Sourceless Forth can work!
  • Structure makes vocabularies nicer
  • Less is more: color, blocks, limits on dictionary.
  • Caution about the challenges of Forth adoption

ROM disassembly
User Manual

flagxor.com
slides
code

⚘

Thank you