Intel mcs-48 manual Instruction SET

INSTRUCTION SET

point for all data transfers within the 8048. Data can be transferred between the 8 registers of each working register bank and the accumulator directly, i.e. the source or destination register is specified by the instruction. The remaining locations of the internal RAM array are referred to as Data Memory and are addressed indirectly via an address stored in either RO or R1 ofthe active working register bank. RO and R1 are also used to indirectly address external data memory when it is present. Transfers to and from internal RAM require one cycle while transfers to external RAM require two. Constants stored in Program Memory can be loaded directly to the accumulator and to the 8 working registers. Data can also be transfered directly between the accumulator and the on-board timer/counter or the accumulator and the Program Status word (PSW). Writing to the PSW alters machine status accordingly and provides a means of restoring status after an interrupt or of altering the stack pointer if necessary.

Accumulator Operations

Immediate data, data memory, or the working registers can be added with or without carry to the accumulator. These sources can also be ANDed, ORed, or Exclusive ORed to the accumulator. Data may be moved to or from the accumulator and working registers or data memory. The two values can also be exchanged in a single operation.

In addition, the lower 4 bits of the accumulator can be exchanged with the lower 4-bits of any of the internal RAM locations. This instruction, along with an instruction which swaps the upper and lower 4-bit halves of the accumulator, provides for easy handling of 4-bit quantities, including BCD numbers. To facilitate BCD arithmetic, a Decimal Adjust instruction is included. This instruction is used to correct the result of the binary addition of two tWO-digit BCD numbers. Performing a decimal adjust on the result in the accumulator produces the required BCD result.

Finally, the accumulator can be: incre- mented. decremented, cleared, or comple- mented and can be rotated left or right 1-bit at a time with or without carry.

Although there is no subtract instruction in the 8048, this operation can be easily implemented with three single-byte single- cycle instructions.

A value may be subtracted from the accumulator with the result in the accumu- lator by:

Complementing the accumulator Adding the value to the accumulator Complementing the accumulator.

Register Operations

The working registers can be accessed via the accumulator as explained above, or can be loaded immediate with constants from program memory. In addition, they can be incremented or decremented or used as loop counters using the decrement and jump, if not zero instruction, as explained under branch instructions.

All Data Memory including working registers can be accessed with indirect instructions via RO and R1 and can be incremented.

Flags

There are four user accessible flags in the 8048: Carry, Auxiliary Carry, FO, and F1. Carry indicates overflow of the accumulator, and Auxiliary Carry is used to indicate overflow between BCD digits and is used during decimal adjust operation. Both Carry and Auxiliary Carry are accessible as part of the program status word and are stored on the stack during subroutines. FO and F1 are undedicated general purpose flags to be used as the programmer desires. Both flags can be cleared or complemented and tested by conditional jump instructions. FO is also accessible via the Program Status word and is stored on the stack with the carry flags.

Branch Instructions

The unconditional jump instruction is two bytes and allows jumps anywhere in the first

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