1. Dr. José M. Reyes Álamo 1

2.  Review: ◦ of Comparisons ◦ of Set on Condition  Statement Labels  Unconditional Jumps  Conditional Jumps

3.  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.

6.  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

10.  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.  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. 1. Transfer control to a label via a jump (goto) instruction 2. Call a label via the CALL instruction, 3. Take the address of a label ◦ Use the address-of operator & ◦ Use the command load effective address lea Syntax: lea( reg 32, Memory_operand );

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

14.  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 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: … jmp laterInPgm; … laterInPgm: …

16.  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 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: > LabelPtr:dword := &stmtLabel; … jmp ( LabelPtr ); … stmtLabel : …

18.  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.  Unconditional jmp provides 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.  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 cmp instruction as the cmp sets the EFLAGS register allowing tests for less than, greater than, equality, etc.

21.  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.

25.  In many instances you will need to generate the opposite of a specific jump instruction. ◦ If the second letter of the jcc instruction is not an “n”, insert an “n” after the “j”. (e.g., je becomes jne and jl becomes jnl) ◦ If the second letter of the jcc instruction is an “n”, then remove that “n” from the instruction. (e.g., jng becomes jg and jne becomes je. ◦ Exception: jpe (jump if parity is even) vs. jpo (jump if parity is odd).

26. C++HLA if (x == y) a++; mov(x, bx) mov(y, cx) cmp( bx, cx ); jne SkipStmts; inc( ax ); SkipStmts:

27. C++HLA mov (0, eax); mov (100, ebx); WhileLabel: cmp(eax, ebx); jnl WhileDone; inc(eax); jmp WhileLabel; WhileDone:  C++ x = 0; while(x <= 100){ x++; }