METHOD AND APPARATUS FOR A BIT FIELD INSTRUCTION

摘要

In a data processor (10) bit fields are used advantageously for efficient use for memory. A bit field instruction is used which can handle bit field widths which are as great as the width of the internal bus of the data processor. In the example explained the internal bus (16) is 32 bits. Furthermore then, the bit field instruction can not only handle bit fields which are contained in 4 bytes or less, but also 5 bytes even though the bus is only 4 bytes wide. This is achieved by accessing four bytes and manipulating it with a switch (401) which has selectable start and end points and with a barrel shifter (402) which has a selectable shift. The portion of the bit field in the remaining fifth byte is also manipulated by the switch (401) and the barrel shifter (402). The end, start, and shift values are derived from offset and width characteristics of the bit field. These two bit field portions are merged to a proper 32 bit format.