CPU and ROM

The preserved general specifications describe a Z80A at 3.25 MHz paired with 2 × 4 KB EPROMs. Period bench tests also describe the ROM as 8 KB and the board as carrying the processor, speaker and ROM chips on the rear portion of the single main PCB.

From a software point of view, the most important consequence is that the Ace boots directly into Forth, not into a BASIC interpreter pretending to be an operating system. The ROM therefore doubles as firmware, language environment and command interface.

Working RAM, dictionary and stacks

The final 1 KB of the internal RAM is not merely "program space". The manual and ROM listing show it split between system variables, the user dictionary, a 12-byte safety gap, the upward-growing data stack and the downward-growing return stack. This layout is very Forth-flavoured and explains why memory upgrades are so valuable once programs become serious.

Screen RAM

The screen is a 24 × 32 character display, so only 768 bytes are needed for the visible screen map. The hardware colour-board article describes 1 KB of RAM storing one byte per character location, with the remainder of the block used by the system as workspace. The review literature also emphasises that this is a proper memory-mapped screen.

Character generator RAM

The Ace's most distinctive hardware trick is that the font is held in RAM rather than ROM. Seven bits from the screen byte select one of 128 character shapes, while three extra address lines pick one of the eight scanlines that make up that character row. Because the character definitions live in RAM, every character can be redefined by the user.

Video logic

The colour-board article is especially useful here because it lays out the video path in plain terms: address counters feed the screen RAM, screen bytes address the character RAM, the selected dot row is loaded into a shift register, and the serialised bitstream becomes raw video before blanking and sync are added. Contemporary reviews confirm that the Ace uses discrete TTL rather than a custom ULA for this work.

Keyboard and sound

The published specs and reviews consistently describe a 40-key rubber keyboard with auto-repeat. Sound is a simple internal speaker or buzzer, CPU-driven rather than handled by a dedicated sound chip. The BEEP word expects pitch and duration values, which reinforces that the firmware is exposing hardware control directly through Forth words.

Main edge connector

The general specs and bench tests both describe a main expansion edge carrying power, address, data and control lines from the CPU. That makes the Ace unusually attractive for RAM packs, printer interfaces and other bolt-on logic because so much of the machine bus is already available at the back.

Secondary video connector

Jupiter's second rear connector is more obscure but arguably more interesting. The colour-board project states that it carries video RAM address, data and write-enable lines plus the composite video signal path. That is how later colour add-ons could track the screen and generate per-character attributes.

Tape and file handling

Mass storage is a domestic cassette recorder at 1500 baud. The 1982 review notes that SAVE, LOAD and VERIFY are built in, that SAVE writes the whole dictionary in RAM to tape, and that a leader tone is written to let the recorder settle before the data begins. This is one of the most practical hardware-facing facts for emulator design because .TAP support maps neatly to the original workflow.

RAM packs and growth

The memory-map documentation states that additional RAM starts at 0x4000, with 16 KB and 48 KB expansions producing totals often described as 19 KB and 51 KB systems once the built-in 3 KB is included. Reviews from the period and the modern hardware scene both make clear that these upgrades were central to the Ace's usefulness.