System Control Flow

System Control Flow

The architectural path followed by commands, modes, permissions, requests, and status conditions as they influence system behavior.

Control flow is the route taken by permission; data flow is the route taken by values.

What is the central distinction between system control flow and system data flow?

Permission

Authorization for a particular state change, transaction start, routing choice, or interpretation behavior.

Control signals do not primarily carry arithmetic values; they determine what the machine is allowed or instructed to do.

What distinguishes control information from datapath information?

Command Flow

The path through which a user action becomes an architecture-level control event.

Mode Flow

The path through which persistent interpretation context is established and distributed.

Execution Flow

The path through which a user execute request becomes a unit-specific enable or START signal.

Status Feedback Flow

The return path through which a local subsystem reports conditions that affect future control decisions.

Next-operation, signed-mode, and execute.

What three major ordinary command paths are traced through the complete arithmetic engine?

btnC.

Which physical button initiates the next-operation control path?

btnR.

Which physical button initiates the signed-mode control path?

btnU.

Which physical button initiates the execute control path?

btnD.

Which physical button is architecturally treated as reset rather than as an ordinary command?

btnC → Debouncer → EdgeDetector → NextOperationPulse → ArithmeticEngineControl → Operation

What is the complete control-flow chain for selecting the next arithmetic operation?

btnR → Debouncer → EdgeDetector → SignedModePulse → ArithmeticEngineControl → SignedMode

What is the complete control-flow chain for changing signed/unsigned interpretation?

btnU → Debouncer → EdgeDetector → ExecuteButtonPulse → ArithmeticEngineControl → ExecutePulse

What is the complete initial control-flow chain for the execute command?

Operation Selection Flow

The command path that changes the persistent identity of the currently selected arithmetic operation.

NextOperationPulse.

Which trusted one-clock event requests a change in Operation?

Operation.

Which persistent control state results from the operation-selection path?

A transient command changes persistent control context.

What temporal transformation occurs from NextOperationPulse to Operation?

Operation persists after the original button event disappears.

Why is the operation-selection path more than merely routing a pulse?

Command Event → Control State

What general control transformation is demonstrated by operation selection?

Signed-Mode Flow

The command path that changes the persistent numerical interpretation context used by compatible arithmetic units.

SignedModePulse.

Which trusted event requests a change in SignedMode?

SignedMode.

Which persistent state records the current signed/unsigned interpretation?

The physical button event is temporary, but SignedMode remains as system context.

What temporal transformation occurs in the signed-mode control path?

Command Event → Interpretation State

What general transformation is demonstrated by signed-mode selection?

Execute Flow

The path through which a transient user execution request is converted into operation-specific local permission.

ExecuteButtonPulse.

Which conditioned input event reaches ArithmeticEngineControl for execution?

ExecutePulse.

Which system-level execution event emerges from ArithmeticEngineControl?

ExecutePulse remains an event rather than becoming persistent configuration state.

How does execution control differ temporally from Operation and SignedMode?

Persistent context + transient event.

What combination determines the meaning of an execution request?

Operation provides context; ExecutePulse provides the moment of requested action.

How do Operation and ExecutePulse cooperate?

Execute Decoding

The control step where ExecutePulse is qualified by Operation and, when necessary, local BUSY status.

Local permission.

What does global ExecutePulse become after execute decoding?

One global execute event is narrowed into a command for one selected stateful unit.

What happens to permission during execute decoding?

Broad Intent → Specific Permission

What general transformation occurs in the execute path?

Control Narrowing

The progressive restriction of a broad system-level command until only the appropriate resource is authorized.

ExecutePulse is broad; AccumulatorEnable is narrow.

Give an example of control narrowing.

ExecutePulse is broad; DividerStart is a resource-specific qualified request.

Give another example of control narrowing.

The closer a control signal gets to a functional unit, the more specific its meaning becomes.

What general pattern occurs as permission flows downward through the hierarchy?

Global Control Context

Operation and SignedMode provide persistent information that gives later events meaning.

Local Control Action

An enable, START, mode input, or other unit-specific signal resulting from global context and system policy.

Operation == ACCUMULATE_OPERATION

What condition identifies the accumulator as the intended execution destination?

ExecutePulse && (Operation == ACCUMULATE_OPERATION)

What permission condition produces AccumulatorEnable?

AccumulatorEnable.

Which local signal grants the ordinary accumulator permission to update?

ExecutePulse && (Operation == WIDE_ACCUMULATE_OPERATION)

What condition produces WideAccumulatorEnable?

WideAccumulatorEnable.

Which signal grants the wide accumulator permission to update?

ExecutePulse && (Operation == MAC_OPERATION)

What condition produces MACEnable?

MACEnable.

Which signal grants the MAC unit permission to update?

ExecutePulse && (Operation == SEQUENTIAL_MULTIPLY) && ~SequentialMultiplyBusy

What qualified permission condition produces SequentialMultiplyStart?

SequentialMultiplyStart.

Which local signal requests permission for a new sequential multiplication transaction?

ExecutePulse && (Operation == DIVIDE_OPERATION) && ~DividerBusy

What qualified permission condition produces DividerStart?

DividerStart.

Which local signal requests permission for a new divider transaction?

Operation Qualification

Requiring the selected Operation to match a particular resource before that resource receives execution permission.

Why is Operation qualification necessary?

Because one ExecutePulse should not cause every stateful arithmetic resource to act.

Resource Qualification

Requiring a local resource to be in an acceptable status before a request becomes a legal local command.

~SequentialMultiplyBusy

Give an example of resource qualification.

~DividerBusy

Give another example of resource qualification.

Operation answers "Is this the intended resource?"; BUSY answers "Can this resource accept the request now?"

How do operation qualification and resource qualification differ?

Intent + legality.

What two ideas combine to create a valid guarded START?

User intent alone does not guarantee local action.

What important lesson follows from BUSY qualification?

Control Acceptance

The point at which a requested action becomes a legal local command.

ExecutePulse can exist without SequentialMultiplyStart.

How can a request exist without being accepted?

The sequential multiplier may already be BUSY.

Why might ExecutePulse fail to become SequentialMultiplyStart?

The divider may already be BUSY.

Why might ExecutePulse fail to become DividerStart?

Request ≠ Acceptance

What compact distinction should be remembered for guarded transactional control?

The request may be dropped.

What happens in this architecture if ExecutePulse occurs for a busy sequential multiplier or divider?

Drop-While-Busy Policy

The control policy in which a request arriving while a transactional resource is BUSY does not become START and is not automatically queued.

There is no queue or retry mechanism in the shown top-level execute decoding.

What does the current busy-guard policy imply about rejected execute requests?

The user would need another later execute event after the unit becomes available.

What would be required to start a transaction after an execute request was dropped while BUSY?

Control Policy

A system-level rule determining whether and when a request may propagate into a local action.

BUSY guarding is control policy rather than arithmetic algorithm.

How should ~BUSY gating be classified architecturally?

The divider algorithm determines HOW division proceeds; top-level gating determines WHETHER a new division may begin.

How do local algorithm and system control policy differ?

The multiplier algorithm determines HOW multiplication progresses; top-level control determines WHETHER START is issued.

Give the same distinction for the sequential multiplier.

Hierarchical Permission

Authorization that passes through multiple control layers rather than directly from the physical input to the datapath.

Physical button → trusted pulse → global event → decoded local command.

What chain demonstrates hierarchical permission?

A physical button does not directly own a datapath state update.

What is the key benefit of hierarchical permission?

Each layer adds meaning, timing discipline, or legality checks.

Why is permission refined across several stages?

Control Abstraction

The replacement of physical-interface details with architecture-level control meanings.

btnU becomes ExecuteButtonPulse and then ExecutePulse.

How does the execute command become progressively more abstract?

The accumulator receives enable, not knowledge of a physical pushbutton.

How does control abstraction protect the accumulator from board-specific details?

The divider receives START, not knowledge of debouncing or edge detection.

How does control abstraction protect the divider from input-conditioning details?

Physical control is translated into semantic control before reaching arithmetic units.

What broad architectural principle is demonstrated?

Control Translation

The conversion of one control vocabulary into another as a command crosses subsystem boundaries.

Button press → command pulse.

Give one input-side example of control translation.

ExecutePulse → AccumulatorEnable.

Give one datapath-side example of control translation.

ExecutePulse → DividerStart.

Give one transactional control-translation example.

Operation → Subtractor.

Give one mode-decoding example of control translation.

Subtractor = (Operation == SUBTRACT_OPERATION)

What top-level equation translates Operation state into CLA add/subtract mode?

Subtractor control is mode decoding, not execute decoding.

How should the Subtractor signal be classified?

Changing Operation can change add/subtract behavior without ExecutePulse.

Why is Subtractor not generated from ExecutePulse?

Control Meaning

The architectural interpretation of a control signal, not merely whether it is 0 or 1.

Different one-bit signals can carry completely different permissions.

Why is bit width insufficient to understand control flow?

ExecutePulse, START, enable, BUSY, and SignedMode may all be one bit yet represent different semantics.

What demonstrates semantic diversity among one-bit control signals?

A correct connection must preserve control meaning as well as width.

What rule should be followed when wiring control signals?

Control Semantic Mismatch

A bug in which electrically compatible signals are connected despite representing different kinds of control information.

Connecting a persistent SignedMode level to a port expecting a one-cycle START pulse.

Give an example of a control semantic mismatch.

Using BUSY as though it were an execute request.

Give another example of a control semantic mismatch.

Control-flow correctness depends on event, level, mode, permission, and status semantics.

What kinds of meaning must be distinguished in control signals?