Building an FPGA 3dfx Voodoo with Modern RTL Tools

Summary

Describing the design is only half of RTL work. The other half is debugging it. The bug that really sold me on this workflow showed up in translucent overlays and text. Most of the frame looked correct, but small clusters of pixels would go mysteriously missing. Because destination-color blending reads the existing framebuffer value, the obvious theory was a memory-ordering bug: a stale read, a read/write hazard, or perhaps the new fill cache occasionally returning old data. Figure 3: Hardware (Mine, left) vs reference (86Box, right). The symptom looked like a framebuffer hazard: a few blended overlay pixels would be lost while most of the frame remained correct. That theory was plausible enough that I chased it hard. I changed write priority, added a true direct no-cache path, and compared alternate-buffer reads. The artifact barely moved. That was the twist. It looked like a framebuffer hazard, but the evidence kept refusing to line up with that explanation. This was where a netlist-aware trace helped much more than a conventional waveform viewer. Instead of staring at a large set of signals and manually aligning them across time, I used conetrace to follow the failing pixels stage by stage through the rasterizer, the TMU, the color-combine logic, and finally the framebuffer output. Once I could trace the suspect pixels end to end, the cache theory collapsed: the wrongness was already present before the framebuffer path could plausibly explain it. Terminal $ conetrace rv path core_1.rasterizer_1.o core_1.writeColor.i_fromPipeline --track 52410001core_1.rasterizer_1.o @ cycle 5241000 payload={x: 396, y: 189, W: 1.972412, S: 124.492, T: 57.031}2-> core_1.tmu_1.io_output @ cycle 5241001 payload={S': 63.469, T': 13.984, lod: 0, texel: 0x58}3-> core_1.fbAccess.read @ cycle 5241002 payload={dst565: 0x4A29}4-> core_1.colorCombine_1.o @ cycle 5241002 payload={src: 0x56C9, dst_blend: 0x4A29, out: 0x39E7}5-> core_1.writeColor.i_fromPipeline @ cycle 5241003 payload={final565...