CET 3510 - Microcomputer Systems Technology Lecture 5

1. CET 3510 - Microcomputer Systems TechnologyLecture 5 Dr. José M. Reyes Álamo

2. Outline • Review: • of Comparisons • of Set on Condition • Statement Labels • Unconditional Jumps • Conditional Jumps

3. Review of Comparison • HLA Syntax: cmp(Left, Right) • Used for comparisons, same as sub, but instead of returning the result only sets certain bits in the flags register. • Z: The zero flag is set if and only if Left = Right. • S:The sign flag is set to one if the result is negative. • O: The overflow flag is set if the difference of Left and Right produced an overflow or underflow. • C:The carry flag is set if subtracting Right from Left requires a borrow.

4. Review of Comparison Syntax

5. Comparison Interpretation

6. Review on Set on Condition • The set on condition (or setcc) instructions set a single byte operand (register or memory location) to zero or one depending on the values in the flags register • These instructions store a zero into the operand if the condition is false, a one if the condition is true • Useful for mapping the result of a comparison to a Boolean value

7. Review Set on Condition Syntax

8. Review Set on Condition Syntax

9. Review Set on Condition Syntax

10. Low Level Control Structures • HLA control structures are similar to C++ and other high-level language • These are NOT true assembly language • Now you will learn how these are represented in real assembly

11. Statement Labels • A low level control structure usually transfers control from one point in your program to another. • Transfer destination is typically specified of using a statement label. • A statement label consists of a valid HLA identifier and a colon. • Don’t have to be declared before you use it • Syntax aLabel:

12. Operations with Labels • Transfer control to a label via a jump (goto) instruction • Call a label via the CALL instruction, • Take the address of a label • Use the address-of operator & • Use the command load effective address lea Syntax: lea( reg32, Memory_operand );

13. Obtaining the Address of a Label program labelDemo; #include( "stdlib.hhf" ); begin labelDemo; lbl1: lea( ebx, lbl1 ); stdout.put( "&lbl1=$", ebx, " &lbl2=", &lbl2, nl ); lbl2: end labelDemo;

14. Unconditional Jump (JMP) • The jmp(jump) instruction unconditionally transfers control to another point in the program. • There are three forms of this instruction one direct jump, and two indirect in the following forms: • jmp label; • jmp( reg32 ); • jmp( mem32 );

15. Direct Jump • Direct jump specifies the target using a label • The label is usually on the same line as an executable instruction or appears on a line preceding an executable machine instruction. • The direct jump instruction is the most commonly used. • Equivalent to a GOTO statement • Syntax: … jmplaterInPgm; … laterInPgm: …

16. Register Indirect Jump • Transfers control to the instruction whose address is specified in the 32-bit general purpose register. • You must load the specified register with the address of some machine instruction prior to the execution of the JMP. • Syntax: … mov(&laterInPgm, ebx); jmp (ebx); … laterInPgm: …

17. Memory Indirect Jump • Memory indirect fetches a dword value from the specified memory location and transfers control to the instruction at the address specified by the contents of the memory location. • Similar to the register indirect JMP except the address appears in a memory location rather than in a register. • Syntax: << statements >> LabelPtr:dword := &stmtLabel; … jmp (LabelPtr); … stmtLabel: …

18. Warning!!! • Low-level JMP instructions can get you into a lot of trouble. • If you do not initialize a register with the address of a valid instruction and you jump indirect through that register, the results are undefined. • If you do not initialize a dword variable with the address of a legal instruction, jumping indirect through that memory location will probably crash your program.

19. Conditional Jump • Unconditional jmpprovides transfer of control but does not allow you to make decisions. • Conditional jumps handle this task. • Conditional jumps are the basic tool for creating loops (i.e. while, for, repeat) and other conditionally executable statements (i.e. if, switch) • Syntax jcc label; cc indicated the type of test you are performing.

20. Conditional Jump Tests • The conditional jumps test one or more flags in the EFLAGS register to see if they match a particular pattern. • If the flags match, the control jumps to the target label. • If the match fails, the CPU ignores the conditional jump and execution continues with the next instruction. • Most of the time, conditional jumps follow the execution of a cmpinstruction as the cmp sets the EFLAGS register allowing tests for less than, greater than, equality, etc.

21. Conditional Jump Limitations • Conditional jump instructions do not provide an indirect form, only allow is a jump to a label in your program. • Conditional jump target label must be within 32,768 bytes of the jump instruction. • This generally corresponds to somewhere between 8,000 and 32,000 machine instructions, it is unlikely you will ever encounter this restriction.

22. Types of Conditional Jump

23. Types of Conditional Jump

24. Types of Conditional Jump

25. Conditional Jump Tips • In many instances you will need to generate the opposite of a specific jump instruction. • If the second letter of the jccinstruction is not an “n”, insert an “n” after the “j”. (e.g., je becomes jne and jlbecomes jnl) • If the second letter of the jccinstruction is an “n”, then remove that “n” from the instruction. (e.g., jngbecomes jgand jnebecomes je. • Exception: jpe(jump if parity is even) vs. jpo(jump if parity is odd).

26. Example of an decision statement if (x == y) a++; mov(x, bx) mov(y, cx) cmp( bx, cx ); jneSkipStmts; inc( ax ); SkipStmts: C++ HLA

27. Example of a loop statement mov (0, eax); mov (100, ebx); WhileLabel: cmp(eax, ebx); jnlWhileDone; inc(eax); jmpWhileLabel; WhileDone: • C++ x = 0; while(x <= 100){ x++; } C++ HLA