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

Vocabularies

Vocabularies

Vocabularies

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