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ISA Draft
StAr is a tagged stack architecture with RISC instruction set and variable instruction length. It is stack-only machine and has no random-access memory(but it has ability to address stack relative to current frame pointer). Instructions are fetched from a kind of pipeline and at this moment there is no support for control flow constructions.
Tagging is an architecture feature that allows dynamic type checking, array bound checking, procedure mechanism support and other useful features. Despite the fact that tag represents high-level concept of data types this functionality is not exposed to programmer.
Initial tagging is being performed automatically with first access to stack entry by instruction. Once tagged value can be checked while performing operations.
Tag is 4bit field with 3 meaning parts:
| 3 | 2 | 1 | 0 |
|---|---|---|---|
| F | A | D | |
- F - this stack entry is marked as frame start
- A - this stack entry is marked as array beginning and its value is array size
- D - data type of stored value.
- U(0) - undefined
- I(1) - integer
- D(2) - float
- A(3) - address
Instructions are byte-aligned and has different size depending on its complexity:
| Type | Size | Byte | Byte | Byte |
|---|---|---|---|---|
| Simple instruction | 1 | Opcode | ||
| Integer instruction | 2 | Opcode | Operand | |
| Float instruction | 3 | Opcode | Operand | |
| Address instruction | 3 | Opcode | Operand | |
Float in StAr is half-precision floating number. It is encoded in 16 bit according to following format:
- 1bit - sign
- 5bit - exponent
- 10bit - fraction
All operations could be roughly divided into five groups:
Here instructions may be represented with I or D postfix, which denotes operand type integer and float correspondingly.
LDC - Load a constant.
| Opcode | Instruction | Description |
|---|---|---|
| 0x01 | LDCI const | Load integer const constant into top of stack. |
| 0x02 | LDCD const | Load float const constant into top of stack. |
LD - load value at constant address.
| Opcode | Instruction | Description |
|---|---|---|
| 0x05 | LDI shift | Load integer value at shift into top of stack. |
| 0x06 | LDD shift | Load float value at shift into top of stack. |
Load a value from relative address inside stack frame into the top of the stack.
LDS - load value at dynamic address.
| Opcode | Instruction | Description |
|---|---|---|
| 0x09 | LDSI | Load integer value at address in top of stack on top of stack. |
| 0x0A | LDSD | Load float value at address in top of stack on top of stack. |
Load a value from relative address inside stack frame specified by top stack value into the top of the stack. Previous value in top of stack is rewritten.
ST - store value at dynamic address
| Opcode | Instruction | Description |
|---|---|---|
| 0x0D | STI | Store integer value from second topmost stack entry by address in top of stack. |
| 0x0E | STD | Store float value from second topmost stack entry by address in top of stack. |
Store a second from top value by relative address inside stack frame specified by top stack value. Top of stack remain unchanged. Previous values in top of stack are rewritten.
ALLOC - allocate frame
| Opcode | Instruction | Description |
|---|---|---|
| 0x20 | ALLOC size | Allocate size bytes in top of stack |
Allocate frame in the top of the stack with given size. All allocated stack entries have U tag.
SCR - drop the top value
| Opcode | Instruction | Description |
|---|---|---|
| 0x24 | SCR | drop the top of stack value. |
MA - mark array head
| Opcode | Instruction | Description |
|---|---|---|
| 0x29 | MAI | Mark a memory unit to be an integers array head. |
| 0x2A | MAD | Mark a memory unit to be a floating point numbers array head. |
There must be an adress at the top of the stack, and an integer below. The instruction pulls address, then pulls the length, and then it marks the corresponding memory unit to be a head of a certain type array with given length.
LDA - load address
| Opcode | Instruction | Description |
|---|---|---|
| 0x43 | LDA shift | load shift relative address into top of stack. |
Load an address into top of the stack. Address is a shift relative to current frame.
INDEX - produce array element address
| Opcode | Instruction | Description |
|---|---|---|
| 0x47 | INDEX | load relative address of array element into top of stack. |
Produce an array element address by its base and index. Base address is obtained by LDA command and is located with index value on the top of the stack. Also this instruction performs array bounds checking. Previous values in top of stack are rewritten.
ADD
| Opcode | Instruction | Description |
|---|---|---|
| 0x81 | ADDI | summarize two topmost stack integer values and replace them with result. |
| 0x82 | ADDD | summarize two topmost stack float values and replace them with result. |
SUB
| Opcode | Instruction | Description |
|---|---|---|
| 0x85 | SUBI | subtract second topmost stack integer value from first and replace them with result. |
| 0x86 | SUBD | subtract second topmost stack float value from first and replace them with result. |
MUL
| Opcode | Instruction | Description |
|---|---|---|
| 0x89 | MULI | multiply two topmost stack integer values and replace them with result. |
| 0x8A | MULD | multiply two topmost stack float values and replace them with result. |
DIV
| Opcode | Instruction | Description |
|---|---|---|
| 0x8D | DIVI | divide first topmost stack integer value on second and place result into top of stack. |
| 0x8E | DIVD | divide first topmost stack float value on second and place result into top of stack. |
Perform integer division if both arguments are integer and replace them with result. Otherwise division result is float.
REM
| Opcode | Instruction | Description |
|---|---|---|
| 0x91 | REM | compute remainder of integer division of first topmost stack value on second and replace them with result. |
CHS
| Opcode | Instruction | Description |
|---|---|---|
| 0x95 | CHSI | change sing of top integer value of stack. |
| 0x96 | CHSD | change sing of top float value of stack. |
FI2D
| Opcode | Instruction | Description |
|---|---|---|
| 0x98 | FI2D | change type of top value of stack from integer to float. |
Change the type of the top element from integer to float.
FD2I
| Opcode | Instruction | Description |
|---|---|---|
| 0x9C | FD2I | change type of top value of stack from float to integer. |
Change the type of the top element from integer to float.
MS
| Opcode | Instruction | Description |
|---|---|---|
| 0x60 | MS | mark top of stack as start of procedure arguments. |
Prepare to enter to a procedure. Mark stack by pushing to it a special element, designating a separate area of arguments.
CALL
| Opcode | Instruction | Description |
|---|---|---|
| 0x64 | CALL | call predefined procedure. |
Call a system hard-coded function, such as print, sin, cos etc.
| call 0 | sin(x) |
| call 1 | cos(x) |
| call 2 | print all params before MS |