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Make $hp point to immediately after usable memory. #464

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Mar 1, 2023
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Make $hp point to immediately after usable memory. #464

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c95f487
Make $hp point to immediately after usable memory.
adlerjohn Mar 1, 2023
a602597
Update description of hp.
adlerjohn Mar 1, 2023
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42 changes: 21 additions & 21 deletions src/vm/index.md
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Expand Up @@ -35,24 +35,24 @@ FuelVM instructions are exactly 32 bits (4 bytes) wide and comprise of a combina

Of the 64 registers (6-bit register address space), the first `16` are reserved:

| value | register | name | description |
|--------|----------|---------------------|-------------------------------------------------------------------------------|
| `0x00` | `$zero` | zero | Contains zero (`0`), for convenience. |
| `0x01` | `$one` | one | Contains one (`1`), for convenience. |
| `0x02` | `$of` | overflow | Contains overflow/underflow of addition, subtraction, and multiplication. |
| `0x03` | `$pc` | program counter | The program counter. Memory address of the current instruction. |
| `0x04` | `$ssp` | stack start pointer | Memory address of bottom of current writable stack area. |
| `0x05` | `$sp` | stack pointer | Memory address on top of current writable stack area (points to free memory). |
| `0x06` | `$fp` | frame pointer | Memory address of beginning of current call frame. |
| `0x07` | `$hp` | heap pointer | Memory address below the current bottom of the heap (points to free memory). |
| `0x08` | `$err` | error | Error codes for particular operations. |
| `0x09` | `$ggas` | global gas | Remaining gas globally. |
| `0x0A` | `$cgas` | context gas | Remaining gas in the context. |
| `0x0B` | `$bal` | balance | Received balance for this context. |
| `0x0C` | `$is` | instrs start | Pointer to the start of the currently-executing code. |
| `0x0D` | `$ret` | return value | Return value or pointer. |
| `0x0E` | `$retl` | return length | Return value length in bytes. |
| `0x0F` | `$flag` | flags | Flags register. |
| value | register | name | description |
|--------|----------|---------------------|----------------------------------------------------------------------------------|
| `0x00` | `$zero` | zero | Contains zero (`0`), for convenience. |
| `0x01` | `$one` | one | Contains one (`1`), for convenience. |
| `0x02` | `$of` | overflow | Contains overflow/underflow of addition, subtraction, and multiplication. |
| `0x03` | `$pc` | program counter | The program counter. Memory address of the current instruction. |
| `0x04` | `$ssp` | stack start pointer | Memory address of bottom of current writable stack area. |
| `0x05` | `$sp` | stack pointer | Memory address on top of current writable stack area (points to free memory). |
| `0x06` | `$fp` | frame pointer | Memory address of beginning of current call frame. |
| `0x07` | `$hp` | heap pointer | Memory address below the current bottom of the heap (points to used/oob memory). |
| `0x08` | `$err` | error | Error codes for particular operations. |
| `0x09` | `$ggas` | global gas | Remaining gas globally. |
| `0x0A` | `$cgas` | context gas | Remaining gas in the context. |
| `0x0B` | `$bal` | balance | Received balance for this context. |
| `0x0C` | `$is` | instrs start | Pointer to the start of the currently-executing code. |
| `0x0D` | `$ret` | return value | Return value or pointer. |
| `0x0E` | `$retl` | return length | Return value length in bytes. |
| `0x0F` | `$flag` | flags | Flags register. |

Integers are represented in [big-endian](https://en.wikipedia.org/wiki/Endianness) format, and all operations are unsigned. Boolean `false` is `0` and Boolean `true` is `1`.

Expand Down Expand Up @@ -90,7 +90,7 @@ Then the following registers are initialized (without explicit initialization, a

1. `$ssp = 32 + MAX_INPUTS*(32+8) + size(tx))`: the writable stack area starts immediately after the serialized transaction in memory (see above).
1. `$sp = $ssp`: writable stack area is empty to start.
1. `$hp = VM_MAX_RAM - 1`: the heap area begins at the top and is empty to start.
1. `$hp = VM_MAX_RAM`: the heap area begins at the top and is empty to start.

## Contexts

Expand Down Expand Up @@ -162,9 +162,9 @@ Whenever memory is written to (i.e. with [`SB`](./instruction_set.md#sb-store-by
If the context is external, the owned memory range is:

1. `[$ssp, $sp)`: the writable stack area.
1. `($hp, VM_MAX_RAM - 1]`: the heap area allocated by this script or predicate.
1. `($hp - 1, VM_MAX_RAM - 1]`: the heap area allocated by this script or predicate.

If the context is internal, the owned memory range for a call frame is:

1. `[$ssp, $sp)`: the writable stack area of the call frame.
1. `($hp, $fp->$hp]`: the heap area allocated by this call frame.
1. `($hp - 1, $fp->$hp - 1]`: the heap area allocated by this call frame.
8 changes: 4 additions & 4 deletions src/vm/instruction_set.md
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Expand Up @@ -804,8 +804,8 @@ All these instructions advance the program counter `$pc` by `4` after performing

Panic if:

- `$hp - $rA` underflows
- `$hp - $rA < $sp`
- `$hp - $rA - 1` underflows
- `$hp - $rA - 1 < $sp`

### CFEI: Extend call frame immediate

Expand All @@ -820,7 +820,7 @@ Panic if:
Panic if:

- `$sp + imm` overflows
- `$sp + imm > $hp`
- `$sp + imm > $hp - 1`

### CFSI: Shrink call frame immediate

Expand Down Expand Up @@ -1223,7 +1223,7 @@ Panic if:
- `$rA + 32` overflows
- `$ssp + $rC > VM_MAX_RAM`
- `$rA + 32 > VM_MAX_RAM`
- `$ssp + $rC > $hp`
- `$ssp + $rC > $hp - 1`
- `$rC > CONTRACT_MAX_SIZE`
- `$rC > MEM_MAX_ACCESS_SIZE`
- Contract with ID `MEM[$rA, 32]` is not in `tx.inputs`
Expand Down