Programmers view

Programmers view

• A programmer

1. writes the program instructions that carryout the desired functionality.
2. may need not to know detailed information about the processors architecture or operation.

• Instead may deal with architectural abstraction.
• The level of abstraction depends on the level of programming.

The Two-levels of programming are

• Assembly language programming.
• Structured-language programming.
• Assembly language programming:- represents processor specific instructions as mnemonics.
• Structured language programming:- uses processor independent instructions.
• A compiler automatically translates processor independent instructions into processor specific instructions.

Instruction set

The assembly language programmer must know the processors instruction set.

• The instruction set describes the bit configurations in IR forms assembly instructions which in turn forms assembly program.
• Instructions are classified into three categories.
• Data transfer instructions:- move data between memory and registers, between input-output channels and registers and between registers themselves.
• Arithmetic Logical instructions:- configure the ALU to carry out a particular function.
• Channel data from the registers to the ALU, ALU to registers.
• Branches can be further categorized as being unconditional jumps, conditional jumps or procedure call and return instructions.
• Unconditional jumps always determine the address of the next instruction.
• While conditional jumps checks the condition to jump to a particular address.
• An instruction set has two parts opcode field, operand field.
• Opcode field:- specifies the operation to take place during the instruction.
• Operand field:- specifies the location of the actual data that takes part in an operation.
• Source operands serve as input to the operation, while a destination operand stores the output.
• The number of operands per instruction varies among processors.
• The operand field may indicate the data’s location through one of several addressing modes.

 Addressing modes

• In immediate addressing, the operand field contains the data itself.
• In register direct addressing, the operand field contains the address of a datapath register in which the data resides.
• In register-indirect addressing, the operand field contains the address of a register, which in turn contains the address of a memory location in which the data resides.
• In direct addressing, the operand field contains the address of a memory location in which the data resides.
• In indirect addressing, the operand field contains the address of a memory location, which in turn contains the address of a memory location in which the data resides.

• Figure 2.6 shows a (trivial) instruction set with 4 data transfer instructions, 2 arithmetic instructions, and 1 branch instruction, for a hypothetical processor.

• Figure 2.7(a) shows a program, written in C, that adds the numbers 1 through 10. Figure 2.7(b) shows that same program written in assembly language using the given instruction set.

    Program and data memory space

• The ES programmer

1. must be aware of the size of program and data memory.
2. must check on chip program and data memory.

• must not exceed program and data memory limits.

    Registers

• The assembly-language programmer

1. Must know how many registers are available for data storage.
2. Must be familiar with registers with special functions.

• Such registers are used for configuring built-in timers, counters, and serial communication devices.

I/O

• The programmer

1. should be aware of the processor’s (I/O).
2. can read or write a port by reading or writing a special register.

• One common I/O facility is parallel I/O.
• Another common I/O facility is a system bus, consists of address and data ports.

Interrupts

• An interrupt causes the proessor to suspend execution of the main program, jumps to an ISR.
• The processor stores the current PC, and sets it to the address of the ISR.
• After executing the ISR control returns to main program by restoring the PC.
• The programmer

1. Should be aware of the types of interrupts.
2. Places each ISR at a specific address in program memory.

 Operating system

• An operating system is

1. a layer of software that provides low-level services to the application layer.
2. decides what program is to run next on the CPU and for how long.

• It does process/task scheduling.

1. It services various H/W interrupts.
2. Implements an environment for management of high-level application programs.

• A system call is the one for an application to invoke the operating system.
• operating system generates a predefined software interrupt required by a program.
• Parameters are typically passed from (to) the application program to (from) the operating system through CPU registers.