code: func SpiChipSelect(chip uint8) {
  	const csMask = 3 // chip select has 2 bits
  
  	cs := uint32(chip & csMask)
  
  	spiMem[csReg] = spiMem[csReg]&^csMask | cs
  }
docs: // SpiChipSelect: Select chip, one of 0, 1, 2
  // for selecting slave on CE0, CE1, or CE2 pin