Witch of Git - jade-mouse/blob - isa.txt

   1 4 16-bit general-purpose registers r0, r1, r2, r3
   2
   3 ALU2: misc misc add sub addpc and or xor
   4 ALU1: inc dec neg comp
   5 ALUA: add
   6 ALUI: lsl lsr asr rol clr set tog ext
   7
   8 7654_3210
   9 --------- ---------
  10 0000_0000           => HALT: halt
  11 0000_0001           => NOPE: nope
  12 000?_????           => RESERVED
  13 0ooo_aabb           => ALU2: a = a op b
  14 1000_aabb           => JALR: jalr ra  # when a == b
  15 1000_aabb           => MOVE: a = b    # when a != b
  16 1001_aaoo           => ALU1: a = op a
  17 101f_aabb           => LDST: a <-> [b]
  18 110o_aabb oiii_iiii => BRNC: branch if a op b  # when a != b
  19 1100_0000 iiii_iiii => JUMP: jump offset
  20 1101_aabb fiii_iiii => LSI0: a <-> [imm * 2]  # a == b
  21 1110_aa00 iiii_iiii => ALUA: a = a + imm
  22 1110_aa01 oooo_iiii => ALUI: a = a op imm
  23 1110_aa10 iiii_iiii => LDLI: lo(a) = imm
  24 1110_aa11 iiii_iiii => LDUI: hi(a) = imm
  25 1111_aabb wpsi_iiii => MEM2
  26
  27
  28 7654_3210 wps      (write | push/pop | size)
  29 --------- ---------
  30 1111_aabb 000i_iiii => LDBI
  31 1111_aabb 001i_iiii => LDWI
  32 1111_aabb 100i_iiii => STBI
  33 1111_aabb 101i_iiii => STWI
  34 1111_aabb 010i_iiii => POPB
  35 1111_aabb 011i_iiii => POPW
  36 1111_aabb 110i_iiii => PSHB
  37 1111_aabb 111i_iiii => PSHW
  38
  39
  40 Encoding branch conditions:
  41 We want: eq/ne, test/testn, lt/gt, ule/ugt
  42 We have 2 operand bits, so we can encode eq, test, lt, ult, and get the negations by swapping operands.
  43 For example, if aa < bb, then we use eq, and if bb < aa, we use ne.
  44 This is natural for lt/gt, and the comparison for eq/ne etc. just needs 1 LUT4.
  45
  46 Load/store should use register pairs, and then specific registers numbers can encode whether it should use the larger instructions.
  47
  48 Skip is valuable because it lets you implement wider add/sub with a relatively short instruction sequence, in particular this wide add is 4 bytes not 5.
  49 For instance, you can do:
  50
  51 a1 = a1 + b1
  52 a2 = a2 + b2
  53 branch if b1 ule a1
  54 a2 = a2 + 1
  55
  56 to compute [a2:a1] + [b2:b1]