Add GM_BASE_ASSET_ID (#561)

Closes #559

VM PR: FuelLabs/fuel-vm#700

src/fuel-vm/index.md

@@ -81,6 +81,7 @@ Every time the VM runs, a single monolithic memory of size `VM_MAX_RAM` bytes is
 To initialize the VM, the following is pushed on the stack sequentially:
 
 1. Transaction hash (`byte[32]`, word-aligned), computed as defined [here](../identifiers/transaction-id.md).
+1. Base asset ID (`byte[32]`, word-aligned)
 1. [`MAX_INPUTS`](../tx-format/consensus_parameters.md) pairs of `(asset_id: byte[32], balance: uint64)`, of:
     1. For [predicate estimation](#predicate-estimation) and [predicate verification](#predicate-verification), zeroes.
     1. For [script execution](#script-execution), the free balance for each asset ID seen in the transaction's inputs, ordered in ascending order. If there are fewer than `MAX_INPUTS` asset IDs, the pair has a value of zero.
@@ -89,7 +90,7 @@ To initialize the VM, the following is pushed on the stack sequentially:
 
 Then the following registers are initialized (without explicit initialization, all registers are initialized to zero):
 
-1. `$ssp = 32 + MAX_INPUTS*(32+8) + size(tx))`: the writable stack area starts immediately after the serialized transaction in memory (see above).
+1. `$ssp = 32 + 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`: the heap area begins at the top and is empty to start.
 

src/fuel-vm/instruction-set.md

@@ -2415,13 +2415,14 @@ Panic if:
 
 Read metadata from memory. A convenience instruction to avoid manually extracting metadata.
 
-| name                         | value     | description                     |
-|------------------------------|-----------|---------------------------------|
-| `GM_IS_CALLER_EXTERNAL`      | `0x00001` | Get if caller is external.      |
-| `GM_GET_CALLER`              | `0x00002` | Get caller's contract ID.       |
-| `GM_GET_VERIFYING_PREDICATE` | `0x00003` | Get index of current predicate. |
-| `GM_GET_CHAIN_ID`            | `0x00004` | Get the value of `CHAIN_ID`     |
-| `GM_GET_TX_START`            | `0x00005` | Get transaction start address   |
+| name                         | value     | description                      |
+|------------------------------|-----------|----------------------------------|
+| `GM_IS_CALLER_EXTERNAL`      | `0x00001` | Get if caller is external.       |
+| `GM_GET_CALLER`              | `0x00002` | Get caller's contract ID.        |
+| `GM_GET_VERIFYING_PREDICATE` | `0x00003` | Get index of current predicate.  |
+| `GM_GET_CHAIN_ID`            | `0x00004` | Get the value of `CHAIN_ID`      |
+| `GM_TX_START`                | `0x00005` | Transaction start memory address |
+| `GM_BASE_ASSET_ID`           | `0x00006` | Base asset ID                    |
 
 If `imm == GM_IS_CALLER_EXTERNAL`: