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

   1 ISA by Jordan Rose
   2
   3 Eight completely symmetric general-purpose registers (r0-7)
   4 Special registers: pc, it (accumulator), data1 (segment), data2 (segment), code (segment)
   5
   6 7654_3210
   7
   8 0000_0000          TRAP (invalid)
   9      0001
  10      001x
  11      0100          JABS jump absolute to code[it]
  12      0101          CABS call absolute code[it], it <- return addr, code <- return segment
  13      0110          JOFF jump pc ± it
  14      0111          COFF call pc ± it, it <- return offset (-it + 1)
  15
  16 0000_1aaa          SWAP a <- it, it <- a
  17    1_0aaa          GETR it <- a
  18    1_1aaa          SETR a <- it
  19
  20 0010_0000          GET1 it <- data1
  21      0001          GET2 it <- data2
  22      0010          GETC it <- code
  23      0011               (reserved for another special register)
  24      01xx
  25      1000          SET1 data1 <- it
  26      1001          SET2 data2 <- it
  27      1010          SETC code  <- it
  28      1011               (reserved for another special register)
  29      11xx
  30
  31 0011_0ooo          ALU1 it <- {zero, lsl1, lsr1, asr1, incr, decr, comp, negt} it
  32      1aaa          ISLT "is less than", for testing overflow / carries: it <- (it < a) ? 1 : 0
  33 01oo_oaaa          ALUR it <- it {addr, subr, andr, iorr, xorr, lslr, lsrr, asrr} a
  34
  35 100i_0aaa          LD1U it <- data1[a], then a += i
  36   0i_1aaa          ST1U it <- data1[a], then a -= i
  37   1i_0aaa          LD2U it <- data2[a], then a += i
  38   1i_1aaa          ST2U it <- data2[a], then a -= i
  39
  40 110x_xxxx
  41 1110_xxxx
  42
  43 1111_00oo iiiiiiii ALUI it <- it {andi, iori, xori, (see below)} i
  44      0011 0iiiiiii ALUI it <- it + i (ADDI)
  45      0011 100ooiii ALUI it <- it {roli, lsli, lsri, asri} i
  46      0011 101xxxxx      (probably permanently unused, but you could cram in more small ALU ops?)
  47      0011 11iiiiii ALUI it <- it + (whole field, thus allowing many negative numbers) (ADDI)
  48      0100 iiiiiiii BEZI branch ±i if it == 0
  49      0101 iiiiiiii      (reserved branch; SUB2, BLTI is no faster than ISLT, BEZI)
  50      0110 iiiiiiii JOFI jump to ±i
  51      0111 iiiiiiii COFI call ±i, it <- return offset (-it + 2)
  52      1xxx iiiiiiii      (reserved w/ immediate)
  53
  54
  55 Some dubiously "nice" properties:
  56 - 0 is invalid
  57 - 1111_xxxx takes immediate
  58 - No reg/imm ops, so data line can be the immediate
  59 - Two data segments, because I'm generous like that
  60 - get/set pairs consistently differentiated by bit 5
  61 - jump/call pairs differentiated by bit 0
  62 - The nicest instruction is "ADDR r5"
  63 - Near and far calls (okay, this is not nice)
  64
  65
  66 # Half-adder it + r0
  67 ADDR r0
  68 SETR r1
  69 ISLT r0
  70 SWAP r1
  71 # main result in it, carry in r1
  72 # more work to make a full adder though
  73
  74 # Half-subtractor it - r0
  75 SETR r1
  76 ISLT r0
  77 SWAP r1
  78 SUBR r0
  79 # main result in it, carry in r0
  80 # more work to make a full subtracter