Add tentative requirements document to capture requests.

Requirements.md

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+# GARY: Graphics Adapter for Retropixel Yeeting
+
+Tentative requirements and wishlist for an FPGA-based media processor
+for retrocomputing.
+
+## Target hardware/era
+
+The design is intended to feel at home in a retrocomputer with a "late
+16-bit era" feel, like the Sentinel 65X: 16-bit CPU clocked around
+5-15MHz with a wide main bus shared by multiple memory-mapped
+peripherals. It aims for capabilities comparable to early VGA graphics
+and early PC audio.
+
+Initially, we'll tailor the design to the Sentinel 65X's needs and
+wants, as a potential replacement for the VERA processor developed for
+the Commander X16.
+
+## Overview
+
+![schematic diagram of a hypothetical 65X computer with GARY. The
+important parts of the schematic are described in text following the
+image.][images/65x-gary-system-overview.png]
+
+This is a general system overview. The 65C816 main processor connects
+to the board's main bus, with peripherals like system RAM and
+expansion ports. GARY also connects to this bus. It has dedicated
+attached video RAM, and provides audio and video output as well as a
+few GPIOs. It also has a dedicated interrupt line, to serve as an
+offboard interrupt controller to the main CPU.
+
+Initially the plan is to mirror the design and features of the VERA
+processor, and modify from there.
+
+![schematic diagram of GARY's graphics rendering pipeline. The system
+bus hooks to a bus interface, which writes data to video RAM and
+configures the other components. Video RAM feeds into two tile engines
+and a sprite engine, their output is merged by a compositor, and
+finally pixels are output by the palette DAC, which also reads palette
+information from video RAM.][images/65x-gary-graphics.png]
+
+## Feature/wish list
+
+Roughly, start from VERA and evolve from there.
+
+- 640x480@60Hz "industry standard" VGA output.
+- RGB color output, 12bpp (4 bits per colour), into a resistor DAC to
+  convert to analog VGA signals.
+- Modifiable 256 color palette.
+- 128KiB video RAM, accessible from CPU.
+- Various tile and sprite modes (same as VERA's, tentatively).
+- Glitch-free video, correct clock domain crossing (unlike VERA).
+- PCM audio with output to I2S DAC. Samples fed directly from CPU.
+- Written in Bluespec SystemVerilog, an high level HDL that makes it
+  hard to screw up timing/synchronization/types. (Not strictly
+  required, but Dave's doing it this way so if you want him to make
+  the thing...)
+- Well tested, good simulator support (again to not go mad trying to
+  debug subtle problems on live hardware).
+
+Desires to better fit 65X's needs than VERA. These mostly rely on having more I/O available, so would require moving up to a low-end Lattice ECP5 instead of a maxed out ICE40.
+
+- Directly expose internal address space on system bus, instead of a
+  narrow window to wider internal space. Full 24 address + 8 data
+  lanes into GARY.
+- Programmable interrupt controller, 8 GPIOs and some timers to
+  trigger interrupts.
+- Absorb the glue logic from the separate CPLD/7400 chips [TODO: what
+  does said logic need to do exactly?]
+
+I want a pony! Things that should be doable, but are more radical
+departures from VERA in various ways. Definitely would need an ECP5.
+
+- Palette in main VRAM, at configurable location (unlike VERA, which
+  has a separate palette memory). Specifically support reconfiguring
+  the palette during frame drawing, to enable cool beam-racing
+  nonsense.
+- Support for palette colour cycling: specify [start:end] range in
+  palette and cycle speed, palette DAC rolls colours by itself.
+- Direct double-buffered framebuffer mode, for GUI stuff. 1bpp and
+  2bpp palettized possible within VERA's RAM budget, more would
+  require larger VRAM.
+- S-Video output in addition to VGA. Require YPbPr conversion to
+  output luma+chroma signals.
+- HDMI or DisplayPort output in addition to VGA. Same
+  resolution/depth, different transport. Requires 8b/10b encoder and
+  additional signals.
+- Support for 65C816's 16-bit move instructions. Normal move sends 8
+  bits every 7 PHI2 clock cycles, 16-bit move sends 2x8 bits on two
+  consecutive PHI2 cycles.
+- Display lists? As in https://www.atariarchives.org/dere/chapt02.php
+  . Effectively a small DSP ISA/very primitive GPU
+  programming. Significant architecture change from VERA.
+- Higher resolution output? Would be nice to hit SVGA resolutions like
+  1024x768 or 1280x1024. Requires gateware running at higher
+  frequency, use of more complex I/O hardware, larger video RAM. Even
+  moreso if we want 24bpp per SVGA, rather than palettized output.