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W9_2: Jumps and loops

Loops are essential programming constructs that allow code to repeat execution. This document discusses different types of loops in assembly language including unconditional loops using jump instructions and conditional loops using conditional jump instructions. It also shows how C-style for loops can be directly translated to equivalent assembly code loops using initialization, condition testing, and updating. Understanding the different loop implementations is important for writing efficient assembly code and dealing with architecture-specific constraints like jump range limits.

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1 Digital Systems and Microprocessor Design (H7068) Daniel Roggen d.roggen@sussex.ac.uk 9.2. Jumps and loops
2 Content • Loops with unconditional jumps • Conditional jumps • Conditional loops • C-style loops to assembler
3 Loop with unconditional jump • Unconditional jumps: changes the value of PC to destination – jmp dst • To do forever a task – Polling-based event loop – sensing-actuation loop • Example: – Read sensors – Compute motor speed – Set motor speed
4 Loop with unconditional jump PC Adr Instr RA RB RC RD -> 00 mov ra,3h 0 0 0 0 02 sub ra,1h 04 jmp 2h 06 ???
5 Loop with unconditional jump PC Adr Instr RA RB RC RD 00 mov ra,3h 3 0 0 0 -> 02 sub ra,1h 04 jmp 2h 06 ???
6 Loop with unconditional jump PC Adr Instr RA RB RC RD 00 mov ra,3h 2 0 0 0 02 sub ra,1h -> 04 jmp 2h 06 ???
7 Loop with unconditional jump PC Adr Instr RA RB RC RD 00 mov ra,3h 2 0 0 0 -> 02 sub ra,1h 04 jmp 2h 06 ???
8 Loop with unconditional jump PC Adr Instr RA RB RC RD 00 mov ra,3h 1 0 0 0 02 sub ra,1h -> 04 jmp 2h 06 ???
9 Loop with unconditional jump PC Adr Instr RA RB RC RD 00 mov ra,3h 1 0 0 0 -> 02 sub ra,1h 04 jmp 2h 06 ???
10 Loop with unconditional jump PC Adr Instr RA RB RC RD 00 mov ra,3h 0 0 0 0 02 sub ra,1h -> 04 jmp 2h 06 ???
11 Loop with unconditional jump PC Adr Instr RA RB RC RD 00 mov ra,3h 0 0 0 0 -> 02 sub ra,1h 04 jmp 2h 06 ???
12 Loop with unconditional jump PC Adr Instr RA RB RC RD 00 mov ra,3h FF 0 0 0 02 sub ra,1h -> 04 jmp 2h 06 ???
13 Loop with unconditional jump PC Adr Instr RA RB RC RD 00 mov ra,3h FF 0 0 0 -> 02 sub ra,1h 04 jmp 2h 06 ???
14 Loop with unconditional jump PC Adr Instr RA RB RC RD 00 mov ra,3h FE 0 0 0 02 sub ra,1h -> 04 jmp 2h 06 ???
15 Loop with unconditional jump PC Adr Instr RA RB RC RD 00 mov ra,3h FE 0 0 0 -> 02 sub ra,1h 04 jmp 2h 06 ???
16 Loop with unconditional jump PC Adr Instr RA RB RC RD 00 mov ra,3h FE 0 0 0 -> 02 sub ra,1h 04 jmp 2h 06 ??? This line is never executed!
17 Conditional jumps • Conditional jumps: changes the value of PC if a condition is met. • Condition is tested by checking the flags (carry, zero). • Flags are set by a prior comparison • JA: jump if above – Jumps if Zero=0 and Carry=0 • JB: jump if below – Jumps if Zero=0 and Carry=1 • JE: jump if equal – Jumps if Zero=1 • And the opposite: JNA, JNB, JNE (not above, not below, not equal)
18 Loop with conditional jump (jne) • Loop with variable from startvalue to 0 (inclusive)
19 Loop with conditional jump (jne) PC Adr Instr RA RB RC RD FLAGS 0 0 0 0 -> 00 mov ra,3h 02 sub ra,1h 04 cmp ra,0h 06 jne 02 08 ???
20 Loop with conditional jump (jne) PC Adr Instr RA RB RC RD FLAGS 3 0 0 0 00 mov ra,3h -> 02 sub ra,1h 04 cmp ra,0h 06 jne 02 08 ???
21 Loop with conditional jump (jne) PC Adr Instr RA RB RC RD FLAGS 2 0 0 0 00 mov ra,3h 02 sub ra,1h -> 04 cmp ra,0h 06 jne 02 08 ???
22 Loop with conditional jump (jne) PC Adr Instr RA RB RC RD FLAGS 2 0 0 0 00 mov ra,3h 02 sub ra,1h 04 cmp ra,0h -> 06 jne 02 08 ???
23 Loop with conditional jump (jne) PC Adr Instr RA RB RC RD FLAGS 2 0 0 0 00 mov ra,3h -> 02 sub ra,1h 04 cmp ra,0h 06 jne 02 08 ???
24 Loop with conditional jump (jne) PC Adr Instr RA RB RC RD FLAGS 1 0 0 0 00 mov ra,3h 02 sub ra,1h -> 04 cmp ra,0h 06 jne 02 08 ???
25 Loop with conditional jump (jne) PC Adr Instr RA RB RC RD FLAGS 1 0 0 0 00 mov ra,3h 02 sub ra,1h 04 cmp ra,0h -> 06 jne 02 08 ???
26 Loop with conditional jump (jne) PC Adr Instr RA RB RC RD FLAGS 1 0 0 0 00 mov ra,3h -> 02 sub ra,1h 04 cmp ra,0h 06 jne 02 08 ???
27 Loop with conditional jump (jne) PC Adr Instr RA RB RC RD FLAGS 0 0 0 0 00 mov ra,3h 02 sub ra,1h -> 04 cmp ra,0h 06 jne 02 08 ???
28 Loop with conditional jump (jne) PC Adr Instr RA RB RC RD FLAGS 0 0 0 0 Z 00 mov ra,3h 02 sub ra,1h 04 cmp ra,0h -> 06 jne 02 08 ???
29 Loop with conditional jump (jne) PC Adr Instr RA RB RC RD FLAGS 0 0 0 0 Z 00 mov ra,3h 02 sub ra,1h 04 cmp ra,0h 06 jne 02 -> 08 ???
30 Loop with conditional jump (jne) PC Adr Instr RA RB RC RD FLAGS 2 0 0 0 Z 00 mov ra,3h 02 sub ra,1h 04 cmp ra,0h 06 jne 02 -> 08 ??? Program continues execution
31 C to assembler loops • In C the syntax for a for loop is: • for(<initialization>;<condition>; <update>) {code} • Example: for(i=3; i != 0; i--) {....} – i will take the value: 3, 2, 1. • Can be easily translated to assembler
32 C to assembler loops: alternative 1 • for(<initialization>;<condition>; <update>) {code} initialization test: test condition if condition then jump to loopcode jump to endofloop loopcode: code update jump to test endofloop: rest of program unconditional conditional
33 C to assembler loops: alternative 2 • for(<initialization>;<condition>; <update>) {code} initialization test: test condition if not condition then jump to endofloop code update jump to test endofloop: rest of program unconditional conditional This requires to negate the condition! Processors usually provide conditional jumps if condition (je,ja,jb) and conditional jumps if not condition (jne,jna,jnb)
34 C to assembler loops • With alternative 1 the conditional jump is to a nearby address (instruction skip) • Alternative 2 leads to more compact code • Some processors have "relative jumps" that allow to change PC by an offset – On Intel/AMD x86 the "short relative jump" allows to offset PC by up to -128 to +127 bytes • What happens on x86 if the loop code is longer than 127 bytes? – Alternative 2 cannot be used with short relative jump! – Alternative 1 must be used Programming influenced by processor architecture! That is why higher level languages (e.g. C) and compilers are used: they can optimize the code Programming influenced by processor architecture! Higher level languages (e.g. C) and compilers allow to select the right assembler construct to optimize the code
35 C to assembler loops • for(i=3; i != 0; i--) {....} • Alternative 2
36 for(i=3; i != 0; i--) {....} PC Adr Instr RA RB RC RD FLAGS 0 0 0 0 -> 00 mov ra,3h 02 cmp ra,0h 04 je 0eh 06 ... 08 ... 0A sub ra,1h 0C jmp 02 0E ???
37 for(i=3; i != 0; i--) {....} PC Adr Instr RA RB RC RD FLAGS 3 0 0 0 00 mov ra,3h -> 02 cmp ra,0h 04 je 0eh 06 ... 08 ... 0A sub ra,1h 0C jmp 02 0E ???
38 for(i=3; i != 0; i--) {....} PC Adr Instr RA RB RC RD FLAGS 3 0 0 0 00 mov ra,3h 02 cmp ra,0h -> 04 je 0eh 06 ... 08 ... 0A sub ra,1h 0C jmp 02 0E ???
39 for(i=3; i != 0; i--) {....} PC Adr Instr RA RB RC RD FLAGS 3 0 0 0 00 mov ra,3h 02 cmp ra,0h 04 je 0eh -> 06 ... 08 ... 0A sub ra,1h 0C jmp 02 0E ???
40 for(i=3; i != 0; i--) {....} PC Adr Instr RA RB RC RD FLAGS 3 0 0 0 00 mov ra,3h 02 cmp ra,0h 04 je 0eh 06 ... 08 ... -> 0A sub ra,1h 0C jmp 02 0E ???
41 for(i=3; i != 0; i--) {....} PC Adr Instr RA RB RC RD FLAGS 2 0 0 0 00 mov ra,3h 02 cmp ra,0h 04 je 0eh 06 ... 08 ... 0A sub ra,1h -> 0C jmp 02 0E ???
42 for(i=3; i != 0; i--) {....} PC Adr Instr RA RB RC RD FLAGS 2 0 0 0 00 mov ra,3h -> 02 cmp ra,0h 04 je 0eh 06 ... 08 ... 0A sub ra,1h 0C jmp 02 0E ???
43 for(i=3; i != 0; i--) {....} PC Adr Instr RA RB RC RD FLAGS 2 0 0 0 00 mov ra,3h 02 cmp ra,0h -> 04 je 0eh 06 ... 08 ... 0A sub ra,1h 0C jmp 02 0E ???
44 for(i=3; i != 0; i--) {....} PC Adr Instr RA RB RC RD FLAGS 2 0 0 0 00 mov ra,3h 02 cmp ra,0h 04 je 0eh -> 06 ... 08 ... 0A sub ra,1h 0C jmp 02 0E ???
45 for(i=3; i != 0; i--) {....} PC Adr Instr RA RB RC RD FLAGS 2 0 0 0 00 mov ra,3h 02 cmp ra,0h 04 je 0eh 06 ... 08 ... -> 0A sub ra,1h 0C jmp 02 0E ???
46 for(i=3; i != 0; i--) {....} PC Adr Instr RA RB RC RD FLAGS 1 0 0 0 00 mov ra,3h 02 cmp ra,0h 04 je 0eh 06 ... 08 ... 0A sub ra,1h -> 0C jmp 02 0E ???
47 for(i=3; i != 0; i--) {....} PC Adr Instr RA RB RC RD FLAGS 1 0 0 0 00 mov ra,3h -> 02 cmp ra,0h 04 je 0eh 06 ... 08 ... 0A sub ra,1h 0C jmp 02 0E ???
48 for(i=3; i != 0; i--) {....} PC Adr Instr RA RB RC RD FLAGS 1 0 0 0 00 mov ra,3h 02 cmp ra,0h -> 04 je 0eh 06 ... 08 ... 0A sub ra,1h 0C jmp 02 0E ???
49 for(i=3; i != 0; i--) {....} PC Adr Instr RA RB RC RD FLAGS 1 0 0 0 00 mov ra,3h 02 cmp ra,0h 04 je 0eh -> 06 ... 08 ... 0A sub ra,1h 0C jmp 02 0E ???
50 for(i=3; i != 0; i--) {....} PC Adr Instr RA RB RC RD FLAGS 1 0 0 0 00 mov ra,3h 02 cmp ra,0h 04 je 0eh 06 ... 08 ... -> 0A sub ra,1h 0C jmp 02 0E ???
51 for(i=3; i != 0; i--) {....} PC Adr Instr RA RB RC RD FLAGS 0 0 0 0 00 mov ra,3h 02 cmp ra,0h 04 je 0eh 06 ... 08 ... 0A sub ra,1h -> 0C jmp 02 0E ???
52 for(i=3; i != 0; i--) {....} PC Adr Instr RA RB RC RD FLAGS 0 0 0 0 00 mov ra,3h -> 02 cmp ra,0h 04 je 0eh 06 ... 08 ... 0A sub ra,1h 0C jmp 02 0E ???
53 for(i=3; i != 0; i--) {....} PC Adr Instr RA RB RC RD FLAGS 0 0 0 0 Z 00 mov ra,3h 02 cmp ra,0h -> 04 je 0eh 06 ... 08 ... 0A sub ra,1h 0C jmp 02 0E ???
54 for(i=3; i != 0; i--) {....} PC Adr Instr RA RB RC RD FLAGS 0 0 0 0 Z 00 mov ra,3h 02 cmp ra,0h 04 je 0eh 06 ... 08 ... 0A sub ra,1h 0C jmp 02 -> 0E ???
55 for(i=3; i != 0; i--) {....} PC Adr Instr RA RB RC RD FLAGS 0 0 0 0 Z 00 mov ra,3h 02 cmp ra,0h 04 je 0eh 06 ... 08 ... 0A sub ra,1h 0C jmp 02 -> 0E ??? Program continues execution
56 C to assembler loops • for(i=3; i != 0; i--) {....} • Alternative 1 -> 00 mov ra,3h 02 cmp ra,0h 04 jne 08h 06 jmp 0E 08 ... 0A sub ra,1h 0C jmp 02 0E ???
57 C to assembler loops • for(i=0; i <3; i++) {....} – i takes the values: 0, 1, 2 • Alternative 1
58 for(i=0; i <3; i++) {....} PC Adr Instr RA RB RC RD FLAGS 0 0 0 0 -> 00 mov ra,0h 02 cmp ra,3h 04 jb 08h 06 jmp 0E 08 ... 0A add ra,1h 0C jmp 02 0E ???
59 for(i=0; i <3; i++) {....} PC Adr Instr RA RB RC RD FLAGS 0 0 0 0 00 mov ra,0h -> 02 cmp ra,3h 04 jb 08h 06 jmp 0E 08 ... 0A add ra,1h 0C jmp 02 0E ???
60 for(i=0; i <3; i++) {....} PC Adr Instr RA RB RC RD FLAGS 0 0 0 0 C 00 mov ra,0h 02 cmp ra,3h -> 04 jb 08h 06 jmp 0E 08 ... 0A add ra,1h 0C jmp 02 0E ???
61 for(i=0; i <3; i++) {....} PC Adr Instr RA RB RC RD FLAGS 0 0 0 0 C 00 mov ra,0h 02 cmp ra,3h 04 jb 08h 06 jmp 0E -> 08 ... 0A add ra,1h 0C jmp 02 0E ???
62 for(i=0; i <3; i++) {....} PC Adr Instr RA RB RC RD FLAGS 0 0 0 0 C 00 mov ra,0h 02 cmp ra,3h 04 jb 08h 06 jmp 0E 08 ... -> 0A add ra,1h 0C jmp 02 0E ???
63 for(i=0; i <3; i++) {....} PC Adr Instr RA RB RC RD FLAGS 1 0 0 0 C 00 mov ra,0h 02 cmp ra,3h 04 jb 08h 06 jmp 0E 08 ... 0A add ra,1h -> 0C jmp 02 0E ???
64 for(i=0; i <3; i++) {....} PC Adr Instr RA RB RC RD FLAGS 1 0 0 0 C 00 mov ra,0h -> 02 cmp ra,3h 04 jb 08h 06 jmp 0E 08 ... 0A add ra,1h 0C jmp 02 0E ???
65 for(i=0; i <3; i++) {....} PC Adr Instr RA RB RC RD FLAGS 1 0 0 0 C 00 mov ra,0h 02 cmp ra,3h -> 04 jb 08h 06 jmp 0E 08 ... 0A add ra,1h 0C jmp 02 0E ??? fast forward som e steps...
66 for(i=0; i <3; i++) {....} PC Adr Instr RA RB RC RD FLAGS 2 0 0 0 C 00 mov ra,0h 02 cmp ra,3h 04 jb 08h 06 jmp 0E 08 ... -> 0A add ra,1h 0C jmp 02 0E ???
67 for(i=0; i <3; i++) {....} PC Adr Instr RA RB RC RD FLAGS 3 0 0 0 C 00 mov ra,0h 02 cmp ra,3h 04 jb 08h 06 jmp 0E 08 ... 0A add ra,1h -> 0C jmp 02 0E ???
68 for(i=0; i <3; i++) {....} PC Adr Instr RA RB RC RD FLAGS 3 0 0 0 C 00 mov ra,0h -> 02 cmp ra,3h 04 jb 08h 06 jmp 0E 08 ... 0A add ra,1h 0C jmp 02 0E ???
69 for(i=0; i <3; i++) {....} PC Adr Instr RA RB RC RD FLAGS 3 0 0 0 Z 00 mov ra,0h 02 cmp ra,3h -> 04 jb 08h 06 jmp 0E 08 ... 0A add ra,1h 0C jmp 02 0E ???
70 for(i=0; i <3; i++) {....} PC Adr Instr RA RB RC RD FLAGS 3 0 0 0 Z 00 mov ra,0h 02 cmp ra,3h 04 jb 08h -> 06 jmp 0E 08 ... 0A add ra,1h 0C jmp 02 0E ???
71 for(i=0; i <3; i++) {....} PC Adr Instr RA RB RC RD FLAGS 3 0 0 0 Z 00 mov ra,0h 02 cmp ra,3h 04 jb 08h 06 jmp 0E 08 ... 0A add ra,1h 0C jmp 02 -> 0E ???
72 for(i=0; i <3; i++) {....} PC Adr Instr RA RB RC RD FLAGS 3 0 0 0 Z 00 mov ra,0h 02 cmp ra,3h 04 jb 08h 06 jmp 0E 08 ... 0A add ra,1h 0C jmp 02 -> 0E ??? Program continues execution
73 Summary • Basic loop constructs can be realized in assembler • Pay attention to the desired range of values of the variables and where the test is placed! • The "C to assembler" examples generalize to more complex tests! – for(i=0; (i<100) && (obstacle1==0); i++) {....} – Use boolean logic to combine multiple simple tests together – Or test individual parts and have several conditional jumps • Knowledge sufficient to complete the coursework assignment involving programming

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W9_2: Jumps and loops

  • 1.
    1 Digital Systems andMicroprocessor Design (H7068) Daniel Roggen d.roggen@sussex.ac.uk 9.2. Jumps and loops
  • 2.
    2 Content • Loops withunconditional jumps • Conditional jumps • Conditional loops • C-style loops to assembler
  • 3.
    3 Loop with unconditionaljump • Unconditional jumps: changes the value of PC to destination – jmp dst • To do forever a task – Polling-based event loop – sensing-actuation loop • Example: – Read sensors – Compute motor speed – Set motor speed
  • 4.
    4 Loop with unconditionaljump PC Adr Instr RA RB RC RD -> 00 mov ra,3h 0 0 0 0 02 sub ra,1h 04 jmp 2h 06 ???
  • 5.
    5 Loop with unconditionaljump PC Adr Instr RA RB RC RD 00 mov ra,3h 3 0 0 0 -> 02 sub ra,1h 04 jmp 2h 06 ???
  • 6.
    6 Loop with unconditionaljump PC Adr Instr RA RB RC RD 00 mov ra,3h 2 0 0 0 02 sub ra,1h -> 04 jmp 2h 06 ???
  • 7.
    7 Loop with unconditionaljump PC Adr Instr RA RB RC RD 00 mov ra,3h 2 0 0 0 -> 02 sub ra,1h 04 jmp 2h 06 ???
  • 8.
    8 Loop with unconditionaljump PC Adr Instr RA RB RC RD 00 mov ra,3h 1 0 0 0 02 sub ra,1h -> 04 jmp 2h 06 ???
  • 9.
    9 Loop with unconditionaljump PC Adr Instr RA RB RC RD 00 mov ra,3h 1 0 0 0 -> 02 sub ra,1h 04 jmp 2h 06 ???
  • 10.
    10 Loop with unconditionaljump PC Adr Instr RA RB RC RD 00 mov ra,3h 0 0 0 0 02 sub ra,1h -> 04 jmp 2h 06 ???
  • 11.
    11 Loop with unconditionaljump PC Adr Instr RA RB RC RD 00 mov ra,3h 0 0 0 0 -> 02 sub ra,1h 04 jmp 2h 06 ???
  • 12.
    12 Loop with unconditionaljump PC Adr Instr RA RB RC RD 00 mov ra,3h FF 0 0 0 02 sub ra,1h -> 04 jmp 2h 06 ???
  • 13.
    13 Loop with unconditionaljump PC Adr Instr RA RB RC RD 00 mov ra,3h FF 0 0 0 -> 02 sub ra,1h 04 jmp 2h 06 ???
  • 14.
    14 Loop with unconditionaljump PC Adr Instr RA RB RC RD 00 mov ra,3h FE 0 0 0 02 sub ra,1h -> 04 jmp 2h 06 ???
  • 15.
    15 Loop with unconditionaljump PC Adr Instr RA RB RC RD 00 mov ra,3h FE 0 0 0 -> 02 sub ra,1h 04 jmp 2h 06 ???
  • 16.
    16 Loop with unconditionaljump PC Adr Instr RA RB RC RD 00 mov ra,3h FE 0 0 0 -> 02 sub ra,1h 04 jmp 2h 06 ??? This line is never executed!
  • 17.
    17 Conditional jumps • Conditionaljumps: changes the value of PC if a condition is met. • Condition is tested by checking the flags (carry, zero). • Flags are set by a prior comparison • JA: jump if above – Jumps if Zero=0 and Carry=0 • JB: jump if below – Jumps if Zero=0 and Carry=1 • JE: jump if equal – Jumps if Zero=1 • And the opposite: JNA, JNB, JNE (not above, not below, not equal)
  • 18.
    18 Loop with conditionaljump (jne) • Loop with variable from startvalue to 0 (inclusive)
  • 19.
    19 Loop with conditionaljump (jne) PC Adr Instr RA RB RC RD FLAGS 0 0 0 0 -> 00 mov ra,3h 02 sub ra,1h 04 cmp ra,0h 06 jne 02 08 ???
  • 20.
    20 Loop with conditionaljump (jne) PC Adr Instr RA RB RC RD FLAGS 3 0 0 0 00 mov ra,3h -> 02 sub ra,1h 04 cmp ra,0h 06 jne 02 08 ???
  • 21.
    21 Loop with conditionaljump (jne) PC Adr Instr RA RB RC RD FLAGS 2 0 0 0 00 mov ra,3h 02 sub ra,1h -> 04 cmp ra,0h 06 jne 02 08 ???
  • 22.
    22 Loop with conditionaljump (jne) PC Adr Instr RA RB RC RD FLAGS 2 0 0 0 00 mov ra,3h 02 sub ra,1h 04 cmp ra,0h -> 06 jne 02 08 ???
  • 23.
    23 Loop with conditionaljump (jne) PC Adr Instr RA RB RC RD FLAGS 2 0 0 0 00 mov ra,3h -> 02 sub ra,1h 04 cmp ra,0h 06 jne 02 08 ???
  • 24.
    24 Loop with conditionaljump (jne) PC Adr Instr RA RB RC RD FLAGS 1 0 0 0 00 mov ra,3h 02 sub ra,1h -> 04 cmp ra,0h 06 jne 02 08 ???
  • 25.
    25 Loop with conditionaljump (jne) PC Adr Instr RA RB RC RD FLAGS 1 0 0 0 00 mov ra,3h 02 sub ra,1h 04 cmp ra,0h -> 06 jne 02 08 ???
  • 26.
    26 Loop with conditionaljump (jne) PC Adr Instr RA RB RC RD FLAGS 1 0 0 0 00 mov ra,3h -> 02 sub ra,1h 04 cmp ra,0h 06 jne 02 08 ???
  • 27.
    27 Loop with conditionaljump (jne) PC Adr Instr RA RB RC RD FLAGS 0 0 0 0 00 mov ra,3h 02 sub ra,1h -> 04 cmp ra,0h 06 jne 02 08 ???
  • 28.
    28 Loop with conditionaljump (jne) PC Adr Instr RA RB RC RD FLAGS 0 0 0 0 Z 00 mov ra,3h 02 sub ra,1h 04 cmp ra,0h -> 06 jne 02 08 ???
  • 29.
    29 Loop with conditionaljump (jne) PC Adr Instr RA RB RC RD FLAGS 0 0 0 0 Z 00 mov ra,3h 02 sub ra,1h 04 cmp ra,0h 06 jne 02 -> 08 ???
  • 30.
    30 Loop with conditionaljump (jne) PC Adr Instr RA RB RC RD FLAGS 2 0 0 0 Z 00 mov ra,3h 02 sub ra,1h 04 cmp ra,0h 06 jne 02 -> 08 ??? Program continues execution
  • 31.
    31 C to assemblerloops • In C the syntax for a for loop is: • for(<initialization>;<condition>; <update>) {code} • Example: for(i=3; i != 0; i--) {....} – i will take the value: 3, 2, 1. • Can be easily translated to assembler
  • 32.
    32 C to assemblerloops: alternative 1 • for(<initialization>;<condition>; <update>) {code} initialization test: test condition if condition then jump to loopcode jump to endofloop loopcode: code update jump to test endofloop: rest of program unconditional conditional
  • 33.
    33 C to assemblerloops: alternative 2 • for(<initialization>;<condition>; <update>) {code} initialization test: test condition if not condition then jump to endofloop code update jump to test endofloop: rest of program unconditional conditional This requires to negate the condition! Processors usually provide conditional jumps if condition (je,ja,jb) and conditional jumps if not condition (jne,jna,jnb)
  • 34.
    34 C to assemblerloops • With alternative 1 the conditional jump is to a nearby address (instruction skip) • Alternative 2 leads to more compact code • Some processors have "relative jumps" that allow to change PC by an offset – On Intel/AMD x86 the "short relative jump" allows to offset PC by up to -128 to +127 bytes • What happens on x86 if the loop code is longer than 127 bytes? – Alternative 2 cannot be used with short relative jump! – Alternative 1 must be used Programming influenced by processor architecture! That is why higher level languages (e.g. C) and compilers are used: they can optimize the code Programming influenced by processor architecture! Higher level languages (e.g. C) and compilers allow to select the right assembler construct to optimize the code
  • 35.
    35 C to assemblerloops • for(i=3; i != 0; i--) {....} • Alternative 2
  • 36.
    36 for(i=3; i !=0; i--) {....} PC Adr Instr RA RB RC RD FLAGS 0 0 0 0 -> 00 mov ra,3h 02 cmp ra,0h 04 je 0eh 06 ... 08 ... 0A sub ra,1h 0C jmp 02 0E ???
  • 37.
    37 for(i=3; i !=0; i--) {....} PC Adr Instr RA RB RC RD FLAGS 3 0 0 0 00 mov ra,3h -> 02 cmp ra,0h 04 je 0eh 06 ... 08 ... 0A sub ra,1h 0C jmp 02 0E ???
  • 38.
    38 for(i=3; i !=0; i--) {....} PC Adr Instr RA RB RC RD FLAGS 3 0 0 0 00 mov ra,3h 02 cmp ra,0h -> 04 je 0eh 06 ... 08 ... 0A sub ra,1h 0C jmp 02 0E ???
  • 39.
    39 for(i=3; i !=0; i--) {....} PC Adr Instr RA RB RC RD FLAGS 3 0 0 0 00 mov ra,3h 02 cmp ra,0h 04 je 0eh -> 06 ... 08 ... 0A sub ra,1h 0C jmp 02 0E ???
  • 40.
    40 for(i=3; i !=0; i--) {....} PC Adr Instr RA RB RC RD FLAGS 3 0 0 0 00 mov ra,3h 02 cmp ra,0h 04 je 0eh 06 ... 08 ... -> 0A sub ra,1h 0C jmp 02 0E ???
  • 41.
    41 for(i=3; i !=0; i--) {....} PC Adr Instr RA RB RC RD FLAGS 2 0 0 0 00 mov ra,3h 02 cmp ra,0h 04 je 0eh 06 ... 08 ... 0A sub ra,1h -> 0C jmp 02 0E ???
  • 42.
    42 for(i=3; i !=0; i--) {....} PC Adr Instr RA RB RC RD FLAGS 2 0 0 0 00 mov ra,3h -> 02 cmp ra,0h 04 je 0eh 06 ... 08 ... 0A sub ra,1h 0C jmp 02 0E ???
  • 43.
    43 for(i=3; i !=0; i--) {....} PC Adr Instr RA RB RC RD FLAGS 2 0 0 0 00 mov ra,3h 02 cmp ra,0h -> 04 je 0eh 06 ... 08 ... 0A sub ra,1h 0C jmp 02 0E ???
  • 44.
    44 for(i=3; i !=0; i--) {....} PC Adr Instr RA RB RC RD FLAGS 2 0 0 0 00 mov ra,3h 02 cmp ra,0h 04 je 0eh -> 06 ... 08 ... 0A sub ra,1h 0C jmp 02 0E ???
  • 45.
    45 for(i=3; i !=0; i--) {....} PC Adr Instr RA RB RC RD FLAGS 2 0 0 0 00 mov ra,3h 02 cmp ra,0h 04 je 0eh 06 ... 08 ... -> 0A sub ra,1h 0C jmp 02 0E ???
  • 46.
    46 for(i=3; i !=0; i--) {....} PC Adr Instr RA RB RC RD FLAGS 1 0 0 0 00 mov ra,3h 02 cmp ra,0h 04 je 0eh 06 ... 08 ... 0A sub ra,1h -> 0C jmp 02 0E ???
  • 47.
    47 for(i=3; i !=0; i--) {....} PC Adr Instr RA RB RC RD FLAGS 1 0 0 0 00 mov ra,3h -> 02 cmp ra,0h 04 je 0eh 06 ... 08 ... 0A sub ra,1h 0C jmp 02 0E ???
  • 48.
    48 for(i=3; i !=0; i--) {....} PC Adr Instr RA RB RC RD FLAGS 1 0 0 0 00 mov ra,3h 02 cmp ra,0h -> 04 je 0eh 06 ... 08 ... 0A sub ra,1h 0C jmp 02 0E ???
  • 49.
    49 for(i=3; i !=0; i--) {....} PC Adr Instr RA RB RC RD FLAGS 1 0 0 0 00 mov ra,3h 02 cmp ra,0h 04 je 0eh -> 06 ... 08 ... 0A sub ra,1h 0C jmp 02 0E ???
  • 50.
    50 for(i=3; i !=0; i--) {....} PC Adr Instr RA RB RC RD FLAGS 1 0 0 0 00 mov ra,3h 02 cmp ra,0h 04 je 0eh 06 ... 08 ... -> 0A sub ra,1h 0C jmp 02 0E ???
  • 51.
    51 for(i=3; i !=0; i--) {....} PC Adr Instr RA RB RC RD FLAGS 0 0 0 0 00 mov ra,3h 02 cmp ra,0h 04 je 0eh 06 ... 08 ... 0A sub ra,1h -> 0C jmp 02 0E ???
  • 52.
    52 for(i=3; i !=0; i--) {....} PC Adr Instr RA RB RC RD FLAGS 0 0 0 0 00 mov ra,3h -> 02 cmp ra,0h 04 je 0eh 06 ... 08 ... 0A sub ra,1h 0C jmp 02 0E ???
  • 53.
    53 for(i=3; i !=0; i--) {....} PC Adr Instr RA RB RC RD FLAGS 0 0 0 0 Z 00 mov ra,3h 02 cmp ra,0h -> 04 je 0eh 06 ... 08 ... 0A sub ra,1h 0C jmp 02 0E ???
  • 54.
    54 for(i=3; i !=0; i--) {....} PC Adr Instr RA RB RC RD FLAGS 0 0 0 0 Z 00 mov ra,3h 02 cmp ra,0h 04 je 0eh 06 ... 08 ... 0A sub ra,1h 0C jmp 02 -> 0E ???
  • 55.
    55 for(i=3; i !=0; i--) {....} PC Adr Instr RA RB RC RD FLAGS 0 0 0 0 Z 00 mov ra,3h 02 cmp ra,0h 04 je 0eh 06 ... 08 ... 0A sub ra,1h 0C jmp 02 -> 0E ??? Program continues execution
  • 56.
    56 C to assemblerloops • for(i=3; i != 0; i--) {....} • Alternative 1 -> 00 mov ra,3h 02 cmp ra,0h 04 jne 08h 06 jmp 0E 08 ... 0A sub ra,1h 0C jmp 02 0E ???
  • 57.
    57 C to assemblerloops • for(i=0; i <3; i++) {....} – i takes the values: 0, 1, 2 • Alternative 1
  • 58.
    58 for(i=0; i <3;i++) {....} PC Adr Instr RA RB RC RD FLAGS 0 0 0 0 -> 00 mov ra,0h 02 cmp ra,3h 04 jb 08h 06 jmp 0E 08 ... 0A add ra,1h 0C jmp 02 0E ???
  • 59.
    59 for(i=0; i <3;i++) {....} PC Adr Instr RA RB RC RD FLAGS 0 0 0 0 00 mov ra,0h -> 02 cmp ra,3h 04 jb 08h 06 jmp 0E 08 ... 0A add ra,1h 0C jmp 02 0E ???
  • 60.
    60 for(i=0; i <3;i++) {....} PC Adr Instr RA RB RC RD FLAGS 0 0 0 0 C 00 mov ra,0h 02 cmp ra,3h -> 04 jb 08h 06 jmp 0E 08 ... 0A add ra,1h 0C jmp 02 0E ???
  • 61.
    61 for(i=0; i <3;i++) {....} PC Adr Instr RA RB RC RD FLAGS 0 0 0 0 C 00 mov ra,0h 02 cmp ra,3h 04 jb 08h 06 jmp 0E -> 08 ... 0A add ra,1h 0C jmp 02 0E ???
  • 62.
    62 for(i=0; i <3;i++) {....} PC Adr Instr RA RB RC RD FLAGS 0 0 0 0 C 00 mov ra,0h 02 cmp ra,3h 04 jb 08h 06 jmp 0E 08 ... -> 0A add ra,1h 0C jmp 02 0E ???
  • 63.
    63 for(i=0; i <3;i++) {....} PC Adr Instr RA RB RC RD FLAGS 1 0 0 0 C 00 mov ra,0h 02 cmp ra,3h 04 jb 08h 06 jmp 0E 08 ... 0A add ra,1h -> 0C jmp 02 0E ???
  • 64.
    64 for(i=0; i <3;i++) {....} PC Adr Instr RA RB RC RD FLAGS 1 0 0 0 C 00 mov ra,0h -> 02 cmp ra,3h 04 jb 08h 06 jmp 0E 08 ... 0A add ra,1h 0C jmp 02 0E ???
  • 65.
    65 for(i=0; i <3;i++) {....} PC Adr Instr RA RB RC RD FLAGS 1 0 0 0 C 00 mov ra,0h 02 cmp ra,3h -> 04 jb 08h 06 jmp 0E 08 ... 0A add ra,1h 0C jmp 02 0E ??? fast forward som e steps...
  • 66.
    66 for(i=0; i <3;i++) {....} PC Adr Instr RA RB RC RD FLAGS 2 0 0 0 C 00 mov ra,0h 02 cmp ra,3h 04 jb 08h 06 jmp 0E 08 ... -> 0A add ra,1h 0C jmp 02 0E ???
  • 67.
    67 for(i=0; i <3;i++) {....} PC Adr Instr RA RB RC RD FLAGS 3 0 0 0 C 00 mov ra,0h 02 cmp ra,3h 04 jb 08h 06 jmp 0E 08 ... 0A add ra,1h -> 0C jmp 02 0E ???
  • 68.
    68 for(i=0; i <3;i++) {....} PC Adr Instr RA RB RC RD FLAGS 3 0 0 0 C 00 mov ra,0h -> 02 cmp ra,3h 04 jb 08h 06 jmp 0E 08 ... 0A add ra,1h 0C jmp 02 0E ???
  • 69.
    69 for(i=0; i <3;i++) {....} PC Adr Instr RA RB RC RD FLAGS 3 0 0 0 Z 00 mov ra,0h 02 cmp ra,3h -> 04 jb 08h 06 jmp 0E 08 ... 0A add ra,1h 0C jmp 02 0E ???
  • 70.
    70 for(i=0; i <3;i++) {....} PC Adr Instr RA RB RC RD FLAGS 3 0 0 0 Z 00 mov ra,0h 02 cmp ra,3h 04 jb 08h -> 06 jmp 0E 08 ... 0A add ra,1h 0C jmp 02 0E ???
  • 71.
    71 for(i=0; i <3;i++) {....} PC Adr Instr RA RB RC RD FLAGS 3 0 0 0 Z 00 mov ra,0h 02 cmp ra,3h 04 jb 08h 06 jmp 0E 08 ... 0A add ra,1h 0C jmp 02 -> 0E ???
  • 72.
    72 for(i=0; i <3;i++) {....} PC Adr Instr RA RB RC RD FLAGS 3 0 0 0 Z 00 mov ra,0h 02 cmp ra,3h 04 jb 08h 06 jmp 0E 08 ... 0A add ra,1h 0C jmp 02 -> 0E ??? Program continues execution
  • 73.
    73 Summary • Basic loopconstructs can be realized in assembler • Pay attention to the desired range of values of the variables and where the test is placed! • The "C to assembler" examples generalize to more complex tests! – for(i=0; (i<100) && (obstacle1==0); i++) {....} – Use boolean logic to combine multiple simple tests together – Or test individual parts and have several conditional jumps • Knowledge sufficient to complete the coursework assignment involving programming