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Added table of VM instructions.

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README.md
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@ -701,6 +701,8 @@ The EightBall Virtual machine has the following features:
The evaluation stack is used for all computations. The VM offers a variety of instructions for maniplating the evaluation stack. All calculations, regardless of the type of the variables involved, is performed using 16 bit arithmetic.
For shorthand, we define the names `X`, `Y`, `Z`, `T` for the top four slots in the evaluation stack. This notation is stolen from the world of HP RPN calculators.
The call stack is used for all memory allocation within the virtual machine, as follows:
- Global variables
- Subroutine parameters
@ -710,7 +712,65 @@ The call stack is used for all memory allocation within the virtual machine, as
### VM Instructions
...
| Instruction | Description |
|-------------|---------------------------------------------------------------------|
| END | Terminate execution |
| LDI | Pushes the following 16 bit word to the evaluation stack |
| LDAW | Replaces X with 16 bit value pointed to by X. |
| LDAB | Replaces X with 8 bit value pointed to by X. |
| STAW | Stores 16 bit value Y in addr pointed to by X. Drops X and Y. |
| STAB | Stores 8 bit value Y in addr pointed to by X. Drops X and Y. |
| LDRW | Replaces X with 16 bit value pointed to by `X+FP+1`. |
| LDRB | Replaces X with 8 bit value pointed to by `X+FP+1`. |
| STRW | Stores 16 bit value Y in addr pointed to by `X+FP+1`. Drops X and Y.|
| STRB | Stores 8 bit value Y in addr pointed to by `X+FP+1`. Drops X and Y. |
| SWP | Swaps X and Y |
| DUP | Duplicates X -> X, Y |
| DUP2 | Duplicates X -> X,Z; Y -> Y,T |
| DROP | Drops X |
| OVER | Duplicates Y -> X,Z |
| PICK | Duplicates stack level specified in `X+1` -> X |
| POPW | Pop 16 bit value from call stack, push onto eval stack [X] |
| POPB | Pop 8 bit value from call stack, push onto eval stack [X] |
| PSHW | Push 16 bit value in X onto call stack. Drop X. |
| PSHB | Push 8 bit value in X onto call stack. Drop X. |
| SPTOFP | Copy stack pointer to frame pointer. (Enter function scope) |
| FPTOSP | Copy frame pointer to stack pointer. (Release local vars) |
| RTOA | Convert FP-relative address in X to absolute address |
| INC | `X = X+1`. |
| DEC | `X = X-1`. |
| ADD | `X = Y+X`. Y is dropped. |
| SUB | `X = Y-X`. Y is dropped. |
| MUL | `X = Y*X`. Y is dropped. |
| DIV | `X = Y/X`. Y is dropped. |
| MOD | `X = Y%X`. Y is dropped . |
| NEG | `X = -X` |
| GT | `X = Y>X`. Y is dropped. |
| GTE | `X = Y>=X`. Y is dropped. |
| LT | `X = Y<X`. Y is dropped. |
| LTE | `X = Y<=X`. Y is dropped. |
| EQL | `X = Y==X`. Y is dropped. |
| NEQL | `X = Y!=X`. Y is dropped. |
| AND | `X = Y&&X`. Y is dropped. |
| OR | `X = Y||X`. Y is dropped. |
| NOT | `X = !X` |
| BAND | `X = Y&X`. Y is dropped. |
| BITOR | `X = Y|X`. Y is dropped. |
| BITXOR | `X = Y^X`. Y is dropped. |
| BITNOT | `X = ~X`. |
| LSH | `X = Y<<X`. Y is dropped. |
| RSH | `X = Y>>X`. Y is dropped. |
| JMP | Jump to address X. Drop X. |
| BRC | If `Y!= 0`, jump to address X. Drop X, Y. |
| JSR | Push PC to call stack. Jump to address X. Drop X. |
| RTS | Pop call stack, jump to the address popped. |
| PRDEC | Print 16 bit decimal in X. Drop X |
| PRHEX | Print 16 bit hex in X. Drop X |
| PRCH | Print character in X. Drop X |
| PRSTR | Print null terminated string pointed to by X. Drop X |
| PRMSG | Print literal string at PC (null terminated) |
| KBDCH | Push character from keyboard onto eval stack |
| KBDLN | Obtain line from keyboard and write to memory pointed to by Y. X contains the max number of bytes in buf. Drop X, Y. | |
### VM Memory Organization