CET 3510 - Microcomputer Systems Technology

Lecture 6Dr. José M. Reyes Álamo

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Review:◦ Statement Labels◦ Unconditional Jumps◦ Conditional Jumps

Outline

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:

Statement Labels

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 leaSyntax:

lea( reg32, Memory_operand );

Operations with Labels

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 );

Unconditional Jump (JMP)

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:…

Direct 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:…

Register 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 :

…

Memory Indirect Jump

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.

Conditional Jump

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.

Conditional Jump Tests

Bit Manipulation

A bit string is a contiguous sequence of one or more bits

A bit string does not have to start or end at any special point. ◦ A bit string could start in bit seven of one byte in

memory and continue through to bit six of the next byte.

◦ A bit string could begin in bit 30 of EAX, and continue from bit zero through 17 of ECX.

In memory, bits must be physically contiguous In registers, the application defines the

continuation register.

Bit String

A bit set is a collection of bits, not necessarily contiguous within some larger data structure. ◦ e.g. 0..3, 7, 12, 24, and 31 from some double

word object forms a set of bits. Usually, we will limit bit sets to some

reasonably sized container object. Normally, container object sizes are about

32 or 64 bits Bit strings are special cases of bit sets.

Bit Set

A bit run is a sequence of bits with all the same value. A run of zeros is a bit string containing all zeros. A run of ones is a bit string containing all ones. The first set bit is the position in a bit string of the

first bit containing a one◦ i.e., the first bit set to ‘1’.

The first clear bit is the position in a bit string of the first bit containing a zero◦ i.e., the first bit set to ‘0’.

The last set bit is the last bit position in a bit string containing a ‘1’; followed by a run of zeros.

The last clear bit is the last bit position in a bit string containing a ‘0’; followed by a run of ones.

Bit Run

A bit offset is the number of bits from some boundary position (usually a byte boundary) to the specified bit.

Bits are numbered starting from zero at the boundary location.

If the offset is less than 32, then the bit offset is the same as the bit number in a byte, word, or double word value.

Bit Offset

A mask is a sequence of bits used to manipulate certain bits in another value.◦ e.g. mask 0000_1111_0000 used with AND, clears

all the bits except bits 4-7◦ mask 0000_1111_0000 used with OR, sets the bits

4-7 and preserves the bits 0-3 and 8-11

Bit Mask

Bit manipulation generally consists of six activities: 1. Setting bits2. Clearing bits3. Inverting bits4. Testing and comparing bits5. Extracting bits6. Inserting bits

Bit manipulation instructions:◦ AND◦ OR◦ XOR◦ NOT◦ Shift◦ Rotate

Bit Manipulation

The AND instruction provides the ability to strip away unwanted bits from a bit sequence and replace them with zeros. ◦ e.g. suppose that a bit string consumes bits 12-24

of EAX. We can isolate it by setting all other bits to zero and using the and instruction:and( %1_1111_1111_1111_0000_0000_0000, eax );

Most programs use AND to clear bits that are not part of the desired bit string.

Bit Manipulation: AND

Bit Manipulation: AND

To check if the bits 12-24 of EAX contain $12F3 you could use the following code:

Or a more sophisticated one:

Most of the time the bit string is aligned to position zero:

Bit Manipulation: AND

OR instruction allows you to mask unwanted bits.

OR instruction does not let you clear bits but it allows you to set bits.

The OR instruction is especially useful for inserting a bit set into some other bit string:◦ Clear all the bits surrounding your bit set in the

source operand.◦ Clear all the bits in the destination operand

where you wish to insert the bit set.◦ OR the bit set with the destination operand.

Bit Manipulation: OR

Assume that you have a value in bits 0-12 of EAX that you want to insert into bits 12-24 of EBX without affecting any of the other bits in EBX.

First you strip bits 13-31 EAX; then strip out bits 12-24 in EBX. Next, shift the bits in EAX to positions 12-24. Finally, OR the values of EAX and EBX

Bit Manipulation: OR

Bit Manipulation: OR

Bit Manipulation: OR

Bit Manipulation: OR

When working with bit masks, it is poor software engineering to use literal numeric constants.

You should always create symbolic constants instead to produce code that is easier to read and maintain.

Bit Manipulation: OR

VS.

Bit Manipulation: OR

NOT: Used to invert all the bits. Not very interesting.

XOR: gives you the ability to invert selected bits belonging to a bit set.

XOR lets you manipulate known data in just about any way imaginable.

Can accomplish the same results as with AND or OR, but with two advantages: 1. You are not limited to forcing the field to all zeros or

all ones, you can have combinations; 2. You can manipulate other bits in the destination

operand at the same time.

NOT and XOR

Suppose that you know that one field contains 1010 that you want to force to zero and another field contains 1000 that you wish to increment by one.

You cannot accomplish both operations with a single AND or OR instruction, but you can do this with a single XOR instruction

How? XOR the first field with %1010 and the second field with %0001.

Warning: This trick only works if you know the current value of a bit set within the destination operand.

XOR Example

AND, OR, and XOR affect the EFLAGs register as follows: ◦ These instructions always clear the carry and

overflow flags.◦ These instructions set the sign flag if the result has a

one in the H.O. bit; they clear it otherwise.◦ These instructions set/clear the zero flag depending

on whether the result is zero.◦ These instructions set the parity flag if there are an

even number of set bits in the L.O. byte of the destination operand, clear the parity flag if there are an odd number of one bits in the L.O. byte of the destination operand.

How EFLAGS is affected

Parity is a very simple error detection scheme. The idea was to count the number of set bits in a

character and include an extra bit in the transmission to indicate whether that character contained an even or odd number of set bits.

The receiving end of the transmission would also count the bits and verify that the extra “parity” bit indicated a successful transmission.

The purpose of the parity flag is to help compute the value of this extra bit.

The 80x86 AND, OR, and XOR instructions set the parity bit if the L.O. byte of their operand contains an even number of set bits.

Not used that much any more.

About the Parity Flag

After executing AND/OR/XOR, the zero flag result is one of the most important.

Many programs often reference this flag after the AND instruction.

Intel added the instruction TEST that logically AND two results and set the flags without affecting either instruction operand.

Bit Manipulation: TEST

There are three main uses of the zero flag after executing AND or TEST 1. Checking to see if a particular bit in an operand

is set.2. Checking to see if at least one of several bits in

a bit set is one.3. Checking to see if an operand is zero.

Bit Manipulation: TEST

Many of these instructions to manipulate bit will set the carry flag:◦ RCL, RCR◦ JC, JNC◦ SETC, SETNC

These are rarely used but are available if needed.

Carry Flag as a Bit Accumulator