Compaq ECQD2KCTE manual Memory Instruction Format, Memory Format Instructions with a Function Code

3.3.1 Memory Instruction Format

The Memory format is used to transfer data between registers and memory, to load an effec- tive address, and for subroutine jumps. It has the format shown in Figure 3–1.

Figure 3–1: Memory Instruction Format

31 26 25 21 20 16 15 0

Opcode

Ra

Rb

Memory_disp

A Memory format instruction contains a 6-bit opcode field, two 5-bit register address fields, Ra and Rb, and a 16-bit signed displacement field.

The displacement field is a byte offset. It is sign-extended and added to the contents of register Rb to form a virtual address. Overflow is ignored in this calculation.

The virtual address is used as a memory load/store address or a result value, depending on the specific instruction. The virtual address (va) is computed as follows for all memory format instructions except the load address high (LDAH):

va ← {Rbv + SEXT(Memory_disp)}

For LDAH the virtual address (va) is computed as follows:

va ← {Rbv + SEXT(Memory_disp*65536)}

3.3.1.1 Memory Format Instructions with a Function Code

Memory format instructions with a function code replace the memory displacement field in the memory instruction format with a function code that designates a set of miscellaneous instruc- tions. The format is shown in Figure 3–2.

Figure 3–2: Memory Instruction with Function Code Format

31 26 25 21 20 16 15 0

Opcode

Ra

Rb

Function

The memory instruction with function code format contains a 6-bit opcode field and a 16-bit function field. Unused function codes produce UNPREDICTABLE but not UNDEFINED results; they are not security holes.

There are two fields, Ra and Rb. The usage of those fields depends on the instruction. See Sec- tion 4.11.

Instruction Formats 3–11