CSE141L Official Dewey Decimal Instruction Set

The following instruction set architecture is provided as a backup, in case your group's ISA in Lab 3 looked problematic. If you got an "OK" or "Good" on Lab 3, please use your own instruction set.

There are four general-purpose registers, and two condition code bits, "O" and "Z".

The instructions are sumarized in the table below.

9  8  7  6  5  4  3  2  1  0  Instruction  Description  Condition bits (Z, O) 
0  0  0  r1  r2  unused  add r1,r2  r1 = r1 + r2  Set/Reset O,Z 
    1  r1  r2  unused  sub r1,r2  r1 = r1 - r2  Set/Reset O,Z 
  1  0  r1  #n  addi r1,#n  r1 = r1 + #n  Set/Reset O,Z 
    1  r1  #n  subi r1,#n  r1 = r1 - #n  Set/Reset O,Z 
1  0  0  r1  r2  #n  ld r1,#n(r2)  r1 = mem[r2 + #n]  O,Z not affected 
    1  r1  r2  #n  st #n(r2),r1  mem[r2 + #n] = r1  O,Z not affected 
  1  0  r1  #n  lui r1,#n  r1[5..9]=#n, r1[4..0]=0  O,Z not affected 
    1  0  0  #n  brno Label  branch if no overflow  Z not affected, O Reset 
        1  #n  brnz Label  branch if not zero  Z Reset, O not affected 
      1  0  unused  pause  Pause  Z,O not affected 
        1  r  unused  jmp r  PC = r  Z, O not affected 


The "#n" notation denotes an immediate operand. Thus, for instance, "010 10 00101" is an "addi" instruction (since the opcode is "010"), and it adds 5 (which is "00101" in binary) to register 2 (since bits 6 and 5 are "10").

The "lui" instruction loads the leftmost 5 bits in the designated register with the immediate operand, and it sets the rightmost 5 bits to zero. The branch instructions ("brno" and "brnz") are PC relative. The 5-bit offset is interpreted as 5 bit signed number, allowing a branch of up to 15 instructions forward or -16 backward. Note: The offset should be added to the incremented PC. Thus, using an offset of zero will result in the next instruction being executed, no matter how the condition code bit ("O" or "Z") is set.

The overflow bit ("O") will be set whenver the result of ALU instruction is overflow, and will be reset when no overflow occurs for an ALU instruction. The zero bit ("Z") will be set whener the result of an ALU instruction is zero and reset when a result is not zero. The "brno" instruction will reset the overflow bit, and "brnz" will reset the zero bit. Other instructions do not affect condition bits.