Appendix F: 8086 Instruction Set Reference Data 679 CLC (no operands) Clear carry flag (no operands) 2 CLC Operands Clocks Transfers' Bytes - 1 ODITSZAPC 0 Flags Coding Example CLC I JCXZ short-label Jump if CX is zero JCXZ Operands Clocks Transfers' Bytes short-label 18or8 - 2 JE/JZ short-label Jump If equallJump if zero The 16-bit JCXZ instruction isn't available (its opcode is used for JRCXZ instead). ARPL (Adjust Requested Privilege Level) isn't available; its opcode is reused for MOVSXD. Instructions that load descriptors and tables have new extended data formats; The behaviour of SYSCALL and SYSRET is substantially different. Microprocessor Microcontroller 8086 These instructions are used to transfer/branch the instructions during an execution. There are two types of branching instructions. The unconditional branch and conditional branch. The Unconditional Program execution transfer instructions are as follows. jcxz rel8: 9+m,5: Jump short if CX register is 0: jcxz rel8_5: E3 cb: jecxz rel8: 9+m,5: . rel16/32 indicates that these instructions map to two; one with a 16-bit relative displacement, the other with a 32-bit relative displacement, depending on the operand-size attribute of the instruction. Virtual 8086 Mode Exceptions The Conditional types of instruction test a certain condition such as the flags (CF, PF, SF, ZF, OF, ), on testing for equality/non-equality, above or less etc. and then jump to a new Specified address. The execution the program then starts from the new address. The Different Conditional instructions are given below. Conditional LOOP instructions: JCXZ/JECXZ - Jump if Register (E)CX is Zero . Usage: JCXZ label JECXZ label (386+) Jumps by default are within -32768 to 32767 bytes from the instruction following the jump. NEAR and SHORT jumps cause the IP to be updated while FAR jumps cause CS and IP to be updated. Clocks; operands: 286: 386: 486: rel8 (relative) 7+m : Loop Instructions in 8086: loop instructions are used to simplify the decrementing, testing and branching portion of the loop. In the above case this portion required two instructions, but in more complicated situation may require more than two instructions. where D8 is byte displacement from current IP. The loop instruction is simplified to: Write an 8086 program which print "hello world" 10 times using JCXZ instruction. Question: Write an 8086 program which print "hello world" 10 times using JCXZ instruction. CX is 0 Description The jcxz instruction tests the contents of the CX or ECX register for 0. jcxz differs from other conditional jumps that it tests the flags, JCXZ is useful at the beginning of a conditional loop that terminates with a conditional loop instruction (such as LOOPNE TARGET LABEL). The JCXZ prevents entering the loop with CX or ECX equal to zero, which would cause the loop to execute 64K or 32G times instead of zero times. Flags Affected None Protected Mode Exceptions JB - 8086. Short Jump if first operand is Below second operand (as set by CMP instruction). Unsigned. Algorithm: if CF = 1 then jump. Syntax. JB Label. Example. include 'emu8086.inc' ORG 100h MOV AL, 1 CMP AL, 5 JB label1 PRINT 'AL is not below 5' JMP exit label1: PRINT 'AL is below 5' exit: RET. Output / Return Value 1. BRANCHING INSTRUCTIONS IN 8086 1 Presented by: Rabin BK BSc.CSIT 2nd Semester. 2. About Branch Instructions Unconditional branch instructions 1.CALL 2.RET 3.INT 4.INTO 5.IRET 6.JMP Conditional branch instructions 1.JZ/JE label