Shifter

Homebrew discrete-logic CPU

"I call it Vera" - Jayne Cobb

Specs

• up to 20 MHz clock
• 4-stage pipeline (runs 4 threads simultaneously)
• 32x 32b general purpose registers (stack addressed)
• 32b ALU (2x L4C381)
• 32b logarithmic shifter
• 16b * 16b -> 32b single cycle multiplier (32b * 32b -> 32b in 5 cycles)
• 32b popcount/extension/truncation unit
• 32b virtual addresses
• 16x4x128 entry, 2 way set-associative, software filled TLB
  • 4 KiB pages
• 64 MiB physical address space (24b address x 32b data)
  • up to 16 MiB SRAM
  • up to 16 MiB I/O space
  • up to 32 MiB pipeline-private, zero-wait SDRAM

Design Constraints

Since everyone in the homebrew CPU community has their own ideas of what kinds of technology are acceptable to use, here are my self-imposed restrictions and allowances:

• Backplane/card cage construction is preferred
  • This facilitates building and testing smaller parts separately
  • Blinkenlights are cool, but not particularly interesting at MHz frequencies
• 3.3V LVTTL/LVCMOS signalling is preferred
  • Backplane signals must not exceed 3.3V (ignoring overshoot transients)
  • 5V TTL outputs are allowed within a board, when paired with an 5V-tolerant receiver
  • Differential LVPECL/LVDS signalling is allowed for clocks
  • All boards should locally generate their own Vcc from a common 12V supply distributed by the backplane
• SMD footprints are fine
  • >= 0.5mm pitch for QFP/QFN/(T)(S)SOP
  • >= 0.8mm pitch for BGA
• Modern SRAM, dual-port SRAM, and SDRAM is fine to use
• Dedicated adder/multiplier/ALU ASICs are allowed
  • But "not quite microprocessor" parts like Am2901 or Am29116/7 are not allowed (except in peripheral devices)
• FPGAs are not allowed
  • Designing with HDLs feels more like work than fun for me, and all of the FPGA toolchains make me want to pull my hair out
• CPLDs and other programmable logic are allowed only if I have created the entire programming toolchain myself
  • Designing for CPLDs (without HDLs) is extremely similar to designing for discrete logic, but simplifies board layout significantly, and often provides better timing characteristics
• Microcontrollers or other ASICs are allowed for bootstrapping or debugging purposes, or within peripheral devices
  • Absolutely none within the data or control paths of the CPU itself