The Assembly Language of the Box-5 of Computer Architecture | CPSC 5155G, Study notes of Computer Architecture and Organization

Download The Assembly Language of the Box-5 of Computer Architecture | CPSC 5155G and more Study notes Computer Architecture and Organization in PDF only on Docsity! The Assembly Language of the Boz–5 The Boz–5 uses bits 31 – 27 of the IR as a five–bit opcode. Of the possible 32 opcodes, only 26 are implemented. Op-Code Mnemonic Description 00000 HLT Halt the Computer 00001 LDI Load Register from Immediate Operand 00010 ANDI Logical AND Register with Immediate Operand 00011 ADDI Add Signed Immediate Operand to Register 00100 NOP Not Yet Defined – At Present it is does nothing 00101 NOP Not Yet Defined – At Present it is does nothing 00110 NOP Not Yet Defined – At Present it is does nothing 00111 NOP Not Yet Defined – At Present it is does nothing This strange gap in the opcodes caused by not using 4, 5, 6, and 7 is an example of adjusting the ISA to facilitate a simpler design of the control unit. With this grouping, the instructions with a “00” prefix either have no argument or have an immediate argument; in short, no memory reference. More Opcodes Here are the opcodes in the range 8 to 15. Op-Code Mnemonic Description 01000 GET Input from I/O Device Register to Register 01001 PUT Output from Register to I/O Device Register 01010 RET Return from Subroutine 01011 RTI Return from Interrupt (Not Implemented) 01100 LDR Load Register from Memory 01101 STR Store Register into Memory 01110 JSR Call a subroutine. 01111 BR Branch on Condition Code to Address Here we begin to see some structure in the ISA. The “01” prefix is shared by all instructions that generate memory address references. This grouping simplifies the design of the Major State Register, which is an integral part of the control unit. Syntax of the Assembly Language (Part 2) Input / Output Operations Syntax Example GET %Rn, I/O_Reg GET %R2, XX // Load the general-purpose register from // the I/O device register. PUT %Rn, I/O_Reg PUT %R0, YY // Store the contents of register into the // I/O device register. Load/Store Operations The syntax of these is fairly simple. For direct addressing we have the following. Syntax Example LDR %Rn, address LDR %R3, X Loads %R3 from address X STR %Rn, address STR %R0, Z Loads %R0 into address Z This clears M[Z] Syntax of the Assembly Language (Part 3) Branch The syntax of this instruction, and its variants, is quite simple. Syntax Example BR Condition_Code, address BRU 5, W The condition code field determines under which conditions the Branch instruction is executed. The eight possible options are. Condition Code Decimal Binary Action 0 000 Branch Always (Unconditional Jump) 1 001 Branch on negative result 2 010 Branch on zero result 3 011 Branch if result not positive 4 100 Branch if carry–out is 0 5 101 Branch if result not negative 6 110 Branch if result is not zero 7 111 Branch on positive result Syntax of the Assembly Language (Part 4) Unary Register-To-Register 31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 – 0 Op Code Destination Register Source Register 5–bit Shift Count (0 – 31) Not Used Opcode = 10000 LLS Logical Left Shift 10001 LCS Circular Left Shift 10010 RLS Logical Right Shift 10011 RAS Arithmetic Right Shift 10100 NOT Logical NOT (Shift count ignored) NOTES: 1. If (Count Field) = 0, a shift operation becomes a register move. 2. If (Source Register = 0), the operation becomes a clear. 3. Circular right shifts are not supported, because they may be implemented using circular left shifts. 4. The shift count, being a 5 bit number, has values 0 to 31 inclusive. Syntactic Sugar Syntactic Sugar Assembled As Comments CLR %R2 LDI %R2, 0 Clears the register CLW X STR %R0, X Clears the word at address X INC %R2 ADDI %R2, 1 Increments the register DEC %R2 ADDI %R2, -1 Decrements the register NOP LLS %R0, %R0, 0 No-Operation: Does Nothing RCS %R3, %R1, 3 LCS %R3, %R1, 29 Right circular shift by N is the same as left circular by (32 – N). The shift count must be a constant number. DBL %R2 LLS %R2, %R2, 1 Left shift by one is the same as a multiply by 2. MOV %R2, %R3 LSH %R2, %R3, 0 Shift by 0 is a copy. NEG %R4, %R5 SUB %R4, %R0, %R5 Subtract from %R0  0 is the same as negation. Syntactic Sugar (Part 2) Syntactic Sugar Assembled As Comments TST %R1 SUB %R0, %R1, %R0 This compares %R1 to zero by subtracting %R0 from it, discarding the result. CMP %R1, %R2 SUB %R0, %R1, %R2 Determines the sign of(%R1) – (%R2), discarding the result. BRU BR 000 Branch always BLT BR 001 Branch if negative BEQ BR 010 Branch if zero BLE BR 011 Branch if not positive BCO BR 100 Branch if carry out is 0 BGE BR 101 Branch if not negative BNE BR 110 Branch if not zero BGT BR 111 Branch if positive