| mspgcc: A port of the GNU tools to the Texas Instruments MSP430 microcontrollers | ||
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MSP430 instructions have at most two operands, a source and a destination.
All instructions are 16 bits long, followed by at most two optional offsets words, one for each of the source and the destination.
The source operand (or the only operand of one-operand instructions) is specified with 2 addressing mode bits and 4 register select bits:
| 00 nnnn | Rn | Register direct |
| 01 nnnn | offset(Rn) | Register indexed |
| 10 nnnn | @Rn | Register indirect |
| 11 nnnn | @Rn+ | Register indirect with post-increment |
The destination operand in a two-operand instruction has only one addressing mode bit, which selects either register direct or indexed. Register indirect can obviously be faked up with a zero index.
Operand addresses are computed in a simple, sequential way. The C statement
*p++ *= 2; |
add @Rn+,-2(Rn) |
When r0 (the program counter) is used as a base address, indexed mode provides PC-relative addressing. This is, in fact, the usual way that TI's MSP430 assembler accesses operands when a label is referred to.
@r0 just specifies the following instruction word in ROM, but @r0+ specifies that word and skips over it. In other word, an immediate constant! You can just write #1234 and the assembler will specify the addressing mode properly.
r1, the stack pointer, can be used with any addressing mode, but @r1+ always increments by 2 bytes, even on a byte access.
When r2 (the status register) or r3 (the zero register) are specified, the addressing mode bits are decoded specially:
| 00 0010 | r2 | Normal access |
| 01 0010 | &<location> | Absolute addressing. The extension word is used as the address directly. The leading & is TI's way of indicating that the usual PC-relative addressing should not be used. |
| 10 0010 | #4 | This encoding specifies the immediate constant 4. |
| 11 0010 | #8 | This encoding specifies the immediate constant 8. |
| 00 0011 | #0 | This encoding specifies the immediate constant 0. |
| 01 0011 | #1 | This encoding specifies the immediate constant 1. |
| 10 0011 | #2 | This encoding specifies the immediate constant 2. |
| 11 0011 | #-1 | This specifies the all-bits-set constant, -1. |
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