Addressing Modes
With four address mode selection bits (P, R, I and N - indexed, relative, indirect and immediate), NAR 2 instructions can specify 16 different addressing modes but not all make sense in all instructions. In the following table:
- M specifies the 32-bit value (content) of memory location x
- BN specifies the program counter
- p specifies the 16-bit signed parameter at location
- Xi specifies the index register selected by data at location
- f is the "effective value" function used for indirect addressing (see details below):
| Addr. flags | Instruction type | ||||
|---|---|---|---|---|---|
| P | R | I | N | Data | Jump |
| - | - | - | - | M | p |
| - | - | - | N | p | p |
| - | - | I | - | M)] | f(M) |
| - | - | I | N | f(M) | f(M) |
| - | R | - | - | M | BN+p |
| - | R | - | N | BN+p | BN+p |
| - | R | I | - | M)] | f(M) |
| - | R | I | N | f(M) | f(M) |
| P | - | - | - | M | Xi+p |
| P | - | - | N | Xi+p | Xi+p |
| P | - | I | - | M)] | f(M) |
| P | - | I | N | f(M) | f(M) |
| P | R | - | - | M | BN+Xi+p |
| P | R | - | N | BN+Xi+p | BN+Xi+p |
| P | R | I | - | M)] | f(M) |
| P | R | I | N | f(M) | f(M) |
Note 1: "N" (immediate) flag has no effect on jump (flow control) instructions, as the processor can not jump into a specified value, but only to a memory address.
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