EEL 3801 Part IV The Assembler OFFSET Operator

EEL 3801 Part IV The Assembler

OFFSET Operator • Returns address of variable used as operand. • Actually, it represents the offset from the beginning of the segment. • The destination operand must be a 16 -bit register. • When a register holds an address (offset) rather than the value therein, it is a pointer to that variable.

SEG Operator • Returns the segment part of a label’s address. • The segment from which the offset is being displaced. • Not the offset.

Example • Used when the base segment register must be initialized to a new segment. • Assume array is in a segment other than what DS points to: push mov mov. pop ds ; Save contents of DS ax, seg array ; set DS to seg ds, ax ; of array bx, offset array ; set offset ; compute whatever ds ; restore original DS

PTR Operator • Specifies the size of an operand. • Can override an operand’s default size – BYTE PTR (8 -bit) – WORD PTR (16 -bit) – DWORD PTR (32 -bit) – QWORD PTR (64 -bit) – TBYTE PTR (80 -bit)

LABEL Directive • Another way to override a variable’s default type. • Does not allocate more memory • Just redefines an existing variable.

Transfer of Control Instructions • CPU executes instructions sequentially, in the exact order presented. • Occasionally, we want a slightly different order of execution, – so must tell assembler so that it knows what is the next instruction to load and decode prior to execution.

Transfer of Control Instructions (cont. ) • Two types: – Conditional transfers – Unconditional transfers.

Unconditional Transfers • Program branches to new location all the time, unconditionally. • New value loaded into the instruction pointer.

Conditional Transfer • Branching only if certain conditions are true. • CPU interprets true/false condition based on content of CX and Flags registers. Some examples of these instructions:

JMP Instruction • Tells CPU to begin execution at another location. • New location to be identified by label • Label translated by assembler into new address. • If jump is to label in current segment, label’s offset loaded into the IP register. • If jump is to another segment, segment address additionally loaded into CS.

JMP Instruction (cont. ) • Syntax is: JMP SHORT/NEAR PTR/FAR PTR destination where SHORT => destination within – 128 to 127 bytes NEAR PTR => destination in same segment (default) FAR PTR => destination in another segment destination => a label or 32 bit segment-offset address.

JMP Instruction (cont. ) • Can move to just about anywhere, • • • to same procedure, to another segment, to RAM or ROM or completely out of the current program. • Loop based on JMP will never end.

LOOP Instruction • Repeats a block of instructions a specific number of times. • Uses CX as a counter, and decrements it every iteration. • The number of cycles must be loaded into CX prior to the loop taking place. • Format is: LOOP destination

LOOP Instruction (cont. ) • Destination must be from – 128 to 127 bytes away from the current location. • Destination typically behind loop instruction. • When it gets to LOOP, assembler decrements CX by 1. • If not zero, transfers control to destination.

Example #1 mov next: mov int loop cx, 12*80 ah, 2 ; DOS function display char dl, ’A’ 21 h; Call DOS function on ah next

Example #2 sum_an_array: mov ax, 0 mov di, offset intarray mov cx, 4 ; array size read_int: add ax, [di] ; add integer to accum add di, 2 loop read_int intarray dw 200 h, 100 h, 300 h, 600 h

Procedures • It is generally infeasible to write a program consisting of long sequence of instructions. • True in assembly or high level language. • More likely, blocks of code that do something logically related • • complex mathematical calculation, setting up or retrieving a data structure, printing something reading a file.

Procedures (cont) • By grouping and interacting these blocks of code, a program is much easier to write, debug and document. • Moreover, the blocks of code may be reusable either in the same program, or in other programs. • These blocks of code are called procedures, or subroutines.

Procedures (cont) • Procedures or subroutines represent a branching of the program control when a call to a subroutine is reached in the program execution. • The assembler must return to the instruction after the one that called the subroutine.

Procedures (cont) • The subroutine can be called through the instruction CALL followed by the name of the subroutine. • CALL saves address of next instruction to stack, depending on whether near or far call. • This is different from the JMP instruction: – JMP does not require a return to the original address from where it was called.

Procedures (cont) • Subroutines that return a value. • They are typically equated to a label that takes on the value resulting from the function execution. • Procedures do not normally return a value. • PROC and ENDP are the bookends that mark the beginning and end of a procedure.

Procedures (cont) • These directives must include the name of the procedure, which precedes the directive itself. • Procedures must include the RET (return) instruction to pop old value of address from run-time stack. • Procedures must not be overlapped. • See example on page 107 of text book.

Near and Far Procedures • The difference between them is how the assembler remembers where to return to.

Near Call • Calling procedure and called procedure are in the same segment of the program. • Assembler generates machine code for a near call. • Before branching to the subroutine, the CALL instruction preserves the current content of the IP register (the instruction pointer) onto the run-time stack.

Near Call (cont) • Remember that the IP points to the next instruction to be executed. • It then copies the address of the first instruction of the called subroutine into the IP register. • Thus, the CPU follows that sequence of instructions that begins at that address.

Near Call (cont. ) • At the end of the subroutine, the RET instruction pops the old value of IP into it again from the stack. • Execution then resumes right from where the call was made.

Far Call • This happens when the calling and called subroutines are in different segments of the code. • The assembler then does a far call. • Now the CALL instruction preserves both the contents of the IP and the CS register onto the stack. • Uses a RETF instruction instead of RET.

Nested Calls • Calls can be nested. • The CALL instruction keeps the addresses in the run-time stack on a last-in-first-out basis, so there is no problem. • The RET instructions are called in reverse order. • See example on page 113, Figure 5. 7.

Interrupts • There are reasons why the CPU needs to be interrupted from its activity. • Two types of interrupts: – Hardware interrupts – Software interrupts

Hardware Interrupts • Are signals from other parts of the system (hardware) that something needs immediate attention from the CPU. • These signals are generated by the Interrupt Controller, a chip called the 8529 IC. • They allow for important events occurring in the background to be “noticed” by the CPU and immediately acted upon.

Hardware Interrupts - Examples • Examples of hardware interrupts are inputs that would be lost if not read and processed quickly. • Sources of such interrupts are: – the keyboard, – external memory (disk) drives, – system clock.

The Interrupt Flag • Occasionally, however, some processes are highly time-sensitive and do not tolerate any interrupts. • The programmer can allow for this by turning off the ability to interrupt the CPU. • This is done through the interrupt flag, IF.

The Interrupt Flag (cont. ) • When the IF is set, interrupts are enabled (normal value); • When clear, interrupts are disabled. • Instruction CLI clears the interrupt flag (disallows interrupts), • Instruction STI sets the interrupt flag (reenables interrupts).

Software Interrupts • These are not really interrupts at all, but rather calls to outside routines such as BIOS or DOS functions. • The INT instruction is used to introduce such “interrupts”. • format as follows: INT number

The INT Instruction • Calls a routine type identified by a number. • The number can range between 0 and FFh. • The routine type can be one of several, and each type will have its own range of possible functions. • The number indicated as an operand of the INT instruction then is referred to the Interrupt Vector Table (IVT).

The INT Instruction (cont. ) • INT maps all the interrupt numbers to operating system subroutine type. • IVT located in the lowest 1 KB of memory. • Each entry is a 32 -bit segment-offset address, which points to the location where the Operating System subroutine resides. • Executes the routine at that address.

The INT Instruction (cont. ) • The various interrupt types are: • INT 10 h: Video services (cursor position, graphics, scroll) • INT 16 h: Keyboard services (read keyboard and check status) • INT 17 h: Printer services (initialize, print, return printer status). • INT 1 Ah: Time of Day (no. of ticks since machine turned on)

The INT Instruction (cont. ) • INT 1 Ch: User Timer Interrupt (executed 18. 2 times per sec. ) • INT 21 h: DOS services (DOS routines for file handling, memory management, I/O, - DOS function calls) • The instruction IRET (interrupt return) tells the processor to resume execution at the next instruction of the calling program.

DOS Function Calls • Called by INT 21 h interrupt serv. routine • Before the function type executes, it looks in register AH to determine the function number that identifies the subroutine itself (NOT the type of interrupt). • Listed in page 116, figure 5. 9.

DOS Function Calls • These numbers are the ones that must be represented in register AH, as that is where the processor looks to see what the function number is. • These are described in detail in the book. • Please read over.

BIOS Level Video Control Instructions • The BIOS level video control instructions are also listed in detail in the book. • Please read them over.