Brall ANL addressA. ADD Adata. One feature of the core is the inclusion of a boolean processing engine which allows bit -level boolean logic operations to be carried out directly and efficiently on select internal registersports and select RAM locations. The lower addresses may reside onchip. As of [update]new derivatives are still developed by many major chipmakers, and major insteuction suppliers such as IAR SystemsKeil and Altium Tasking continuously release updates.

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Power saving mode on some derivatives One feature of the core is the inclusion of a boolean processing engine which allows bit -level boolean logic operations to be carried out directly and efficiently on select internal registers , ports and select RAM locations.

Another feature is the inclusion of four bank selectable working register sets which greatly reduce the amount of time required to perform the context switches to enter and leave interrupt service routines. With one instruction, the can switch register banks, avoiding the time consuming task of transferring the critical registers to RAM.

The main program then performs serial reads and writes simply by reading and writing 8-bit data to stacks. Derivative features[ edit ] As of [update] , new derivatives are still developed by many major chipmakers, and major compiler suppliers such as IAR Systems , Keil and Altium Tasking continuously release updates.

The original core ran at 12 clock cycles per machine cycle, with most instructions executing in one or two machine cycles. All Silicon Labs , some Dallas and a few Atmel devices have single cycle cores. MCS based microcontrollers have been adapted to extreme environments.

The last digit can indicate memory size, e. Memory architecture[ edit ] The MCS has four distinct types of memory — internal RAM, special function registers, program memory, and external data memory. The is designed as a modified Von-Neumann Architecture with segregated memory Data and Instructions ; it can only execute code fetched from program memory, and has no instructions to write to program memory.

Which is similar to Harvard Architecture. Most systems respect this distinction, and so are unable to download and directly execute new programs. IRAM from 0x00 to 0x7F can be accessed directly, using an 8-bit absolute address that is part of the instruction. The original has only bytes of IRAM. The added IRAM from 0x80 to 0xFF, which can only be accessed indirectly; direct access to this address range goes to the special function registers.

Most clones also have a full bytes of IRAM. The 32 bytes from 0x00—0x1F memory-map the 8 registers R0—R7. They cannot be accessed indirectly via R0 or R1; indirect access to those addresses will access the second half of IRAM. It may be on- or off-chip, depending on the particular model of chip being used. Program memory is read-only, though some variants of the use on-chip flash memory and provide a method of re-programming the memory in-system or in-application.

It can also be on- or off-chip; what makes it "external" is that it must be accessed using the MOVX move external instruction. Registers[ edit ] The only register on an that is not memory-mapped is the bit program counter PC. This specifies the address of the next instruction to execute.

Relative branch instructions supply an 8-bit signed offset which is added to the PC. The stack grows upward; the SP is incremented before pushing, and decremented after popping a value. Gives the parity XOR of the bits of the accumulator, A. User defined, UD. May be read and written by software; not otherwise affected by hardware. Overflow flag , OV. Set when addition produces a signed overflow.

Register select 0, RS0. The low-order bit of the register bank. Set when banks at 0x08 or 0x18 are in use. Register select 1, RS1. The high-order bit of the register bank. Set when banks at 0x10 or 0x18 are in use. Flag 0, F0. Auxiliary carry , AC. Set when addition produces a carry from bit 3 to bit 4. Carry bit , C. Often used as the general register for bit computations, or the "Boolean accumulator".

Accumulator, A 0xE0 This register is used by most instructions. B, register 0xF0 This is used as an extension to the accumulator for multiply and divide instructions. These are the 16 IRAM locations from 0x20—0x2F, and the 16 special function registers 0x80, 0x88, 0x90, Any bit of these bytes may be directly accessed by a variety of logical operations and conditional branches.

For the former, the most significant bit of the accumulator can be addressed directly, as it is a bit-addressable SFR. For the latter, there are explicit instructions to jump on whether or not the accumulator is zero. There is also a two-operand compare and jump operation. Instruction set[ edit ] Instructions are all 1 to 3 bytes long, consisting of an initial opcode byte, followed by up to 2 bytes of operands.

The least significant nibble of the opcode selects the primary operand as follows: x8—xF: Register direct, R0—R7. The most significant nibble specifies the operation as follows. Not all support all addressing modes; the immediate mode in particular is unavailable when the primary operand is written to. Instruction mnemonics use destination, source operand order.

Immediate mode opcode 0x04 specifies the accumulator, INC A. Immediate mode opcode 0x14 specifies the accumulator, DEC A. Immediate mode opcode 0x74 specifies the accumulator, MOV A, data. Immediate mode opcode 0x84 is not used for this operation, as it duplicates opcode 0x This operation borrows and there is no subtract without borrow.

Immediate mode opcode 0xA4 is not used, as immediates serve only as sources. Memory direct mode opcode 0xA5 is not used, as it duplicates 0x Note that there is no compare and jump if equal instruction, CJE. Immediate mode opcode 0xC4 is not used for this operation. Immediate mode opcode 0xD4 , and register indirect mode 0xD6, 0xD7 are not used. Immediate mode is not used for this operation opcode 0xE4 , as is duplicates opcode 0x Immediate mode opcode 0xF4 is not used, as it would have no effect.

The irregular instructions comprise 64 opcodes, having more limited addressing modes, plus several opcodes scavenged from inapplicable modes in the regular instructions.



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x86 instruction listings



Intel MCS-51


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