MASM Padding Conventions

Overview

Padding requirements differ based on procedure invocation type:

Invocation	Padding Required	Input/Output Elements
call	Explicit padding in comments	Exactly 16
exec	No explicit padding	No requirement

Stack Depth Floor: 16

Miden VM enforces a minimum operand-stack depth of 16 elements (MIN_STACK_DEPTH = 16 in the VM core). When an operation would naively shrink the stack below 16, the VM auto-fills the missing positions with zeros via the overflow-table mechanism. The actual depth stays exactly 16; only the visible content shrinks.

This invariant applies at the entry boundary of:

• call procedures,
• note scripts and transaction scripts (entered via dyncall at depth 16).

It does NOT apply to mid-chain exec procedures, which share the caller's stack and can drop the visible count below 16 by consuming caller elements (see Danger Zone).

Tracking the floor in inline comments

When the naive math would put the stack below 16, the # => tracker must reflect the actual auto-padded depth, not the naive count.

# entry at depth 16: [VALUE, pad(12)]
dropw
# => [pad(16)]   # correct: VALUE replaced with zeros, depth still 16

Not:

# => [pad(12)]   # wrong: depth is still 16, only the visible content shrank

This shows up most often at the start of note scripts that don't use their input arguments:

begin
    dropw
    # => [pad(16)]
    ...
end

Call Procedures

Procedures invoked with call must have explicit padding in:

1. Doc comments (#!) for Inputs/Outputs
2. Inline comments (#) showing stack state

Doc Comment Format

Use pad(N) notation where N + other elements = 16:

#! Inputs:  [ASSET, pad(12)]
#! Outputs: [pad(16)]
#!
#! Invocation: call
pub proc receive_asset

Inline Comment Format

Track padding through the procedure:

exec.native_account::set_item
# => [OLD_VALUE, pad(12)]

dropw
# => [pad(16)] auto-padded to 16 elements    

Exec Procedures

Procedures invoked with exec should NOT have explicit padding:

#! Inputs:  [PUB_KEY]
#! Outputs: []
#!
#! Invocation: exec
pub proc authenticate_transaction

Why No Padding for Exec

exec procedures share the caller's stack directly. Explicit padding would be misleading because:

• The actual stack may have additional elements from the caller
• The procedure may consume caller's stack elements

Danger Zone

If an exec procedure's stack falls below the specified stack elements, it will consume stack items from its caller, potentially leading to unexpected behavior. This is a bug and should be fixed by ensuring the procedure maintains sufficient stack depth and avoiding dropping more stack elements than available.

Example of Dangerous Behavior

# => [num_approvers, threshold]
dropw # dropw drops 4 elements, which will result in "negative" stack consumption (consuming 2 elements from the caller's stack)

Intermediate States

Inside a procedure, the stack may temporarily exceed 16 elements:

# => [num_approvers, threshold, MULTISIG_CONFIG, pad(12)]
#     ^--- 18 elements total, must be reduced before return

These extra elements must be explicitly dropped before the procedure returns (directly or via called procedures).

Debugging Stack Depth

When unsure whether the stack matches the depth you expect, use the assembly's debug instructions to inspect it at runtime. These cost zero VM cycles, do not affect the program hash, and are stripped at compile time when the assembler is not in debug mode.

• debug.stack – print the full operand stack.

• debug.stack.N – print only the top N elements (1 ≤ N < 256).

• sdepth – push the current stack depth onto the stack as a felt; useful when you need depth as a runtime value, e.g. to assert it:

  sdepth push.16 eq assert.err="depth must be 16 here"

Run with the --debug flag to see output:

miden-vm run program.masm --debug

Without --debug, debug instructions are silently removed. Remove or comment out debug.* lines before committing production MASM.

Validation Checklist

For all invocation types:

• Inline # => trackers reflect the post-auto-pad depth (never below 16) at boundaries that enforce the floor (call, note scripts, tx scripts)
• No debug.* instruction is left in production MASM

For call procedures:

• Inputs doc comment shows exactly 16 elements with pad(N)
• Outputs doc comment shows exactly 16 elements with pad(N)
• Inline comments use # => format with pad(N) notation
• All intermediate states track the full stack including padding

For exec procedures:

• No pad(N) in Inputs/Outputs doc comments
• No explicit padding in inline stack state comments
• Verify stack never drops below safe depth

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Source: 0xMiden/protocol — distributed by TomeVault.