Intel 8XC251SQ, 8XC251SA, 8XC251SP, 8XC251SB manual Unconditional Jumps, Calls and Returns

PROGRAMMING

5.5.3Unconditional Jumps

There are five unconditional jumps. NOP and SJMP jump to addresses relative to the program counter. AJMP, LJMP, and EJMP jump to direct or indirect addresses.

•NOP (No Operation) is an unconditional jump to the next instruction.

•SJMP (Short Jump) jumps to any instruction within -128 to 127 of the next instruction.

•AJMP (Absolute Jump) changes the lowest 11 bits of the PC to jump anywhere within the current 2-Kbyte block of memory. The address can be direct or indirect.

•LJMP (Long Jump) changes the lowest 16 bits of the PC to jump anywhere within the current 64-Kbyte region.

•EJMP (Extended Jump) changes all 24 bits of the PC to jump anywhere in the 16-Mbyte address space. The address can be direct or indirect.

5.5.4Calls and Returns

The MCS 251 architecture provides relative, direct, and indirect calls and returns.

ACALL (Absolute Call) pushes the lower 16 bits of the next instruction address onto the stack and then changes the lower 11 bits of the PC to the 11-bit address specified by the instruction. The call is to an address that is in the same 2-Kbyte block of memory as the address of the next instruction.

LCALL (Long Call) pushes the lower 16 bits of the next-instruction address onto the stack and then changes the lower 16 bits of the PC to the 16-bit address specified by the instruction. The call is to an address in the same 64-Kbyte block of memory as the address of the next instruction.

ECALL (Extended Call) pushes the 24 bits of the next instruction address onto the stack and then changes the 24 bits of the PC to the 24-bit address specified by the instruction. The call is to an address anywhere in the 16-Mbyte memory space.

RET (Return) pops the top two bytes from the stack to return to the instruction following a sub- routine call. The return address must be in the same 64-Kbyte region.

ERET (Extended Return) pops the top three bytes from the stack to return to the address follow- ing a subroutine call. The return address can be anywhere in the 16-Mbyte address space.

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