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Assembly Language Programming Processor architecture Zbigniew Jurkiewicz, Instytut Informatyki UW October 10, 2017 Zbigniew Jurkiewicz, Instytut Informatyki UW Assembly Language Programming Processor architecture

Processor architecture Also called ISA ( Instruction Set Architecture ). Classification stack machine; accumulator-based; if additional specialized registers present (e.g. address register), then extended accumulator ; register-memory; register-register + load/store. Additionally classifiers take into account the number of instruction arguments (0–3) and the number of memory addresses instructions (0–3). Zbigniew Jurkiewicz, Instytut Informatyki UW Assembly Language Programming Processor architecture

Different privilege levels The simplest version: ordinary ( user ) system ( supervisor ). On Intel processors there are 4 levels (numbered 0–3), but in practice only 2 are used. Zbigniew Jurkiewicz, Instytut Informatyki UW Assembly Language Programming Processor architecture

Memory model Alignment problem — the legal or preferred addresses should be multiple of some basic value. Example: Pentium II has 36-bit addresses, but the address bus is only 33-bit wide. On the bus three lower address bits are always equal 0. Each fetch from memory always gets 8 bytes. Zbigniew Jurkiewicz, Instytut Informatyki UW Assembly Language Programming Processor architecture

Byte order for larger units Two possibilities: little endian : less important (‘lower’) byte first, forces us to read strings byte after byte, otherwise multibyte register will have them in reverse order; big endian : standard for networks. Zbigniew Jurkiewicz, Instytut Informatyki UW Assembly Language Programming Processor architecture

Processor state register Often contains bits setting current working mode, for hardware supporting the privileged mode . Most flags are divided into two categories conditional , they signal the result of the last operation; control , they set parameters for current working mode. Zbigniew Jurkiewicz, Instytut Informatyki UW Assembly Language Programming Processor architecture

Pentium processor flags SF (sign) the highest bit from the result of the last arithmetic-logic operation (so equals 1 when the result was negative); ZF (zero) set when the last result was exactly zero; PF (parity) set when the lowest byte of the last result has even number of ones. CF (carry) set to the value of carry from the highest position for the last operation (important for unsigned numbers), also used in some other situations having nothing to do with arithmetic per se; OF (overflow) set when the last operation resulted in overflow (important for signed numbers); IF (interrupts) for masking interrupts; DF (direction) the order of working for block transfer instructions, 0 means increase the addrresses after each step. Zbigniew Jurkiewicz, Instytut Informatyki UW Assembly Language Programming Processor architecture

Machine instructions Machine instruction general forms byte sequences of varying lengths (“CISC”) words: (nearly) all instructions have the same size (“RISC”) Format of single instruction Divided into fields, the exact number and kind of fields may be different for different groups of instructions. Sometimes preceded by optional prefixes, either changing the instruction semantics or providing additional part of argument address. Attentions: some descriptions (and some assemblers) treat prefixes as separate instructions Zbigniew Jurkiewicz, Instytut Informatyki UW Assembly Language Programming Processor architecture

Machine instructions Fields: Operation code, determines the computation to be done Addressing mode, describes how to find an argument or its effective address. This specifies also a rule for interpretation of address fields, e.g. whether the instruction operates on bytes, words etc. (unless this information is not contained in operation code). Adresses, specifies a register or a cell in memory Constant (so called immediate argument) There are also special fields, e.g. giving number of position shifted for rotations and shifts. Zbigniew Jurkiewicz, Instytut Informatyki UW Assembly Language Programming Processor architecture

Computing effective address Effective address = final address used for memory reference, obtained as a result of processing according to the specified addressing mode. Address space. Segment registers, segment table. Segment descriptors. Zbigniew Jurkiewicz, Instytut Informatyki UW Assembly Language Programming Processor architecture

Instruction set Types of instructions: Arithmetical: addition, subtraction, multiplication, division for integers ADD, ADC, INC, SUB, SBB, DEC, MUL, DIV, CMP Separate arithmetical instructions for other number representations (e.g. BCD, ASCII, floating-point). Arithmetic with ( saturation ): if the result is too big, it is replaced by the maximum possible value. Used for DSP (e.g. MMX instructions on Pentium). Zbigniew Jurkiewicz, Instytut Informatyki UW Assembly Language Programming Processor architecture

Instruction set Boolean operations, arguments interpreted as sequences of bits, the operation is preformed on parallel on all pairs of corresponding bits. NOT, AND, OR, XOR, TEST Rotations and shifts. Circular rotations ROR, ROL, RCR, RCL Shifts: logical and arithmetic (division/multiplication by 2) SHL, SHR, SAL, SAR Zbigniew Jurkiewicz, Instytut Informatyki UW Assembly Language Programming Processor architecture

Instruction set Data transfer (movement). Used to move or change the contents of register/meory cell: MOV, XCHG, MOVZX, MOVSX. Stack operations: PUSH, POP , PUSHF, POPF. Input/output: IN, OUT. Some processors provide special complex operations for repeated sequences of operations, e.g. block transfers or search LODS, STOS, MOVS, SCAS, CMPS, REP Zbigniew Jurkiewicz, Instytut Informatyki UW Assembly Language Programming Processor architecture

Instruction set Control instructions. They change the normal sequential execution of consecutive instructions, modifying the contents of instruction count. The mandatory argument (sometimes there are additional ones) is the address of the next instruction to be executed: given as absoulute or relative. Unconditional jump, always executed (JMP). Branches (conditional jumps): jump happens only when some condition is satisfied. The condition is determined by checking the appropriate processor flag or flags — those are set depending on the result of the last “computational” instruction. JZ/JE, JNZ/JNE, JC, JNC, JO, JNO, JP , JNP Zbigniew Jurkiewicz, Instytut Informatyki UW Assembly Language Programming Processor architecture

Instruction set Separate branches for signed (JG, JGE, JL, JLE) and unsigned (JA, JAE, JB, JBE) numbers. The alternative is to put the result of comparison instruction into some register and later checking this register (e.g. DEC/Alpha processor, but also Pentium). Pro: the jump can be delayed, contra: takes one of registers. The last possibility is compare and branch instruction, which does comparison and possible jump at the same time (e.g. VAX, PA-RISC), these creates problems for aggressive pipeline processors. Zbigniew Jurkiewicz, Instytut Informatyki UW Assembly Language Programming Processor architecture

Instruction set Subroutine call/return CALL, RET Programming interrupts (extracodes) and return from interrupt INT, IRET Loops LOOP , JCXZ, LOOPE, LOOPNE Sometimes also conditional skip instruction – the single instruction is executed or not depending on a condition. Zbigniew Jurkiewicz, Instytut Informatyki UW Assembly Language Programming Processor architecture

Instruction set Manual setting of flags CLI, STI, CLC Do nothing instruction (NOP) — useful filler. Spetial instruction. Allow execution only in priviledged mode. Manipulation of protection mechanism, halting the processor (HLT). Zbigniew Jurkiewicz, Instytut Informatyki UW Assembly Language Programming Processor architecture

Addressing modes Types of instruction arguments. The determine the method of computing argument’s location. Zbigniew Jurkiewicz, Instytut Informatyki UW Assembly Language Programming Processor architecture

Addressing modes implicit (hidden) mul ecx immediate, the argument is contained in the address field mov eax,10 register, the argument is in a register, address fields contains the number of register add eax,ebx Zbigniew Jurkiewicz, Instytut Informatyki UW Assembly Language Programming Processor architecture

Addressing modes direct, the address field contains argument address mov eax,[100] register indirect, the argument address is given in the register specified mov eax,[esi] indirect, the address field contains the address of the memory cell, which contains the final address, rarely used today because consumes too many cycles (does not exist on x86). relative, the address field contains the offset relative to the instruction counter; mostly in conditional jumps jle 30 Zbigniew Jurkiewicz, Instytut Informatyki UW Assembly Language Programming Processor architecture