Repz instruction in 8086

REP, REPE, REPZ F3H REPNE, REPNZ F2H Note that REP and REPE and not distinct Machine (microcode) interpretation of REP and REPE code depends on instruction currently being executed! 8086 Instruction Encoding-15 Immediate Operand Instructions! Immediate mode instructions have only one register or memory operand; the other is encoded in the REPE/REPZ - Repeat Equal / Repeat Zero Usage: REPE REPZ Modifies flags: None Repeats execution of string instructions while CX != 0 and the Zero Flag is set. CX is decremented and the Zero Flag tested after each string operation. The combination of a repeat prefix and a segment override on processors other than the 386 may result in errors if an interrupt occurs before CX=0. Instructions of this group transfer program execution from the normal sequence of instructions to the specified destination or target. The following instructions come under this category: Here, CF = Carry Flag ZF = Zero Flag OF = Overflow Flag SF = Sign Flag CX = Register Flag Manipulation and Processor Control Instructions REP. REP is a prefix written before one of the string instructions. It is used for repeating an instruction count number of times, where count is stored in the CX register. After every operation the CX register is decremented and the zero flag is tested; the process continues till CX = 0. Usage REP . Flags Depend on String Only 8 low bit s of result f8086 instructions Page 28 of 53 are checked. Set by CMP, SUB, ADD, TEST, AND, OR, XOR inst ruct ions. Algorit hm : if PF = 0 then jump Exam ple: include 'emu8086.inc' ORG 100h JPO label MOV AL, 00000111b ; AL = 7 OR AL, 0 ; just set flags. 1 Instruction set of 8086 The instruction set of 8086 can be classified into following groups 1. Data transfer instructions 2. Arithmetic instructions 3. The instruction with REPE / REPZ prefix will be repeated if CX 0 and ZF =1 ( for each iteration CX is automatically decremented by 1). The repeat operation is terminated if CX = 0 or ZF 8086 instructions 1. Instruction Set of 8086 The 8086 instructions are categorized into the following main types. i. Data Copy / Transfer Instructions ii. Arithmetic and Logical Instructions iii. They are meant for use with the CMPS and SCAS instructions. With REPE/REPZ the basic compare or scan operation Microprocessor Microcontroller 8086 String is a group of bytes/words and their memory is always allocated in a sequential order. String is either referred as byte string or word string. Here we will see some instructions which are used to manipulate the string related operations. The String manipulation instructions are as follows. George John It is either referred as byte string or word string. Their memory is always allocated in a sequential order. Instructions used to manipulate strings are called string manipulation instructions. Following is the table showing the list of string manipulation instructions: OPCODE. OPERAND. EXPLANATION. EXAMPLE. Complete 8086 instruction set. Complete 8086 instruction set . Quick reference: AAA AAD AAM AAS ADC ADD AND CALL CBW CLC CLD CLI CMC CMP: CMPSB CMPSW CWD DAA DAS DEC DIV HLT IDIV IMUL IN INC INT INTO IRET JA: JAE JB JBE JC REPZ chain instruction: Repeat following CMPSB, CMPSW, SCASB, SCASW instructions while ZF = 1 (result is Zero), maximum RET -- Return from Procedure Opcode Instruction Clocks Description C3 RET 10+m Return (near) to caller CB RET 18+m,pm=32+m Return (far) to caller, same privilege CB RET pm=68 R