Machine code. Nils Jansen December 12, 2017

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1 Machine code Nils Jansen December 12, 2017

2 Overview So far Circuits Memory ALU Data Path Fetch-Decode-Execute Cycle Fetch-decode-execute cycle Today Machine code Stack 2

4 RUN1718 CPU Fetch-Test-Decode+Execute Several instructions for RUN1718 CPU (in HADES) XOR READ ADD HALT 12

5 Type of Instructions Influence Program Flow HALT Data Transport READ, WRITE, LOADHI Computation ADD, XOR, Stack PUSH, POP 13

6 Machine code: Example 1 (HALT) HALT cond Instruction with condition cond = 1111 Condition is TRUE 14

7 Machine code: Example 2 (LOADHI) LOADHI.NZ 0x3FFFFF, R LOADHI bit const. target register R3: 0011 condition NZ:

8 Systematic Structure for Machine Code instructions of the same type machine codes have similar structures format := group machine codes with similar structure format 1:+0 HALT format 2+3: memory (READ, WRITE) format 4+5: stack operations (PUSH + POP) format 6: LOADHI format 7: computation + JUMP(derived) 16

9 All Formats, binary condition bit constant src reg B dest reg condition bit constant src reg A src reg B condition src reg A condition dest reg condition bit constant dest reg condition 0 opc. f src reg A src reg B dest reg condition 0 opc. f 0 10-bit constant src reg B dest reg condition all instructions: conditional uniform structure makes control unit easier 17

10 Example: Format 2 and 3 (READ/WRITE) bit constant src reg B dest reg condition bit constant src reg A src reg B condition READ: WRITE: Memory address: register B + constant READ: result to destination register WRITE: write value from register A 18

11 Design Considerations: Alternative Format condition 1 1 X X dest reg variabel condition 1 0 dest reg variabel condition 0 dest reg variabel condition also possible less uniform structure more complicated control unit more complicated assembler 19

12 More structure Source registers and constant in the same bits as far as possible General machine code X X X X flg nc signed constant src reg A src reg B dest reg condition 20

13 Example: Format 7 (Computation) X X X X flg nc signed constant src reg A src reg B dest reg condition 0 opcode flg src reg A src reg B dest reg condition 0 opcode flg 0 signed constant src reg B dest reg condition variable part: information regarding operands nc = 1: two registers as input nc = 0: constant and register as input 21

14 Example: Format 7 (Computation) X X X X flg nc signed constant src reg A src reg B dest reg condition 0 opcode flg src reg A src reg B dest reg condition 0 opcode flg 0 signed constant src reg B dest reg condition

15 Example: Format 7 (Computation) Constants are assigned 10 bits of space fits exactly. MOVE, JUMP This is the machine code of - MOVE constant, reg OR constant, R0, reg - JUMP label ADD constant, PC, PC 23

16 Derived Instructions Some assembly commands don t have machine code instructions! Example JUMP MOVE CMPf Translated by the assembler! 24

17 Problem: 32-bits-constants To store a 32-bit number in a register, the machine code must at least contain: 32 bits constant 4 bit target register 4 bit condition 1 bit identification of format 41 bits total but each machine code has only 32 bits! 25

18 Solution: LOADHI (calculation) instruction can set the lowest 10 bits of register to an arbitrary value instruction format 3 for the highest 22 bits: LOADHI constant, dest reg stores constant in the highest 22 bits of the dest register sets lowest 10 bits to 0. use LOADHI + ADD together! 26

19 Solution: LOADHI constant (highest 22b) dest reg condition constant (lowest 10b) dest reg dest reg condition 1027 in R1 LOADHI 1, R1 ADD 3, R1, R in R6 LOADHI 1, R6 ADD -24, R6, R6 27

20 Formats 4 and 5 (PUSH/POP) src reg A condition dest reg condition Stack: PUSH POP 28

21 Stack Stack is a data structure Operations: PUSH puts value on top of stack POP retrieves top value of stack Order: LIFO 29

22 Why a stack? Stack is need for: Function call: call stack Local varibales Computation of expressions (store intermediate results) 30

23 Stack Operations in RUN1718 CPU Part of the main memory is reserved for stack Stack pointer SP indicates 'top' of stack (register R14) PUSH Rn - Decrease SP by 4 - Store value in Rn at address [SP] POP Rm - Read value at address [SP] to Rm - Increase SP by 4 31

24 Function Parameterized piece of code that you want to perform several times from different places in the main program Afterwards continue with the main program, at the place where the call occurred So: put call return address on the stack for call function 33

25 Function Call Call Save parameters (on stack or in registers) Put return address on stack Jump to function address Return Save result function (usually in register) Return address from stack Jump to return address 34

26 Format 4 + 5: PUSH and POP src reg A condition dest reg condition Format 4 is PUSH; how do we implement this? subtract 4 from R14 write from A to new R14 Format 5 is POP: READ from R14 to D add 4 to R14 35

27 Format 4: PUSH src reg A condition dest reg condition 4 is PUSH; can we adapt the machine code such that 4 is no special format? bits := 110 (ADD) bits := (-4) 36

28 PUSH, more efficient bit constant src reg A src reg B condition src reg A condition 0 opc. f src reg A src reg B dest reg condition 0 opc. f 0 10-bit constant src reg B dest reg condition 4 is PUSH; now stack increment is handled by standard logic! bits := 110 (ADD) bits := 00 (no flags, constant) bits := (-4) Careful: reg A is not needed in ADD! 37

29 RISC and CISC assembly languages started at the level of the practical assembler better processors -> more and more powerful instructions (for instance memory access at each instruction and multiple operations in one instruction) but also: increasingly complicated processors RISC = Reduced Instruction Set Computer as counter-movement 38

30 Program Languages Higher programming language: abstract, mathematical notation Assembly: mnemonic instruction names can directly be translated to machine code Machine language: bit patterns in memory hard to read easy to make mistakes 39

31 (Dis-)Advantages of Assembly w.r.t machine language + easier to understand + less error-prone w.r.t higher languages + full control of CPU + efficient, smal programs only for specific CPU-type small instructions error-prone 40

32 Preview: What does an Assembler do? translate mnemonic names compute addresses (often) macros pseudo instructions 41

33 Summary: Machine Code Machine codes have a fixed layout per instruction format for all instructions together Assembler resolves derived instructions 43

34 Summary: Stack Stack: data structure (LIFO) Operations: PUSH, POP Applications: function calls, local variables, calculating expressions 44

How does the CPU execute programs? Nils Jansen December 5, 2017 (Based on slides by Jeroen Keiren and Niek Janssen)

How does the CPU execute programs? Nils Jansen December 5, 2017 (Based on slides by Jeroen Keiren and Niek Janssen) Overview 2 Overview So far Circuits Memory ALU 2 Overview So far Circuits Memory ALU