About Breakpoints

A breakpoint may be set at any location in core. It consists of a set of conditions; when those conditions are satisfied on that line, execution will be halted. The following conditions may be specified:

• Events – Any combination of these can be chosen. Execution will halt when any one of the events that you select happens. Refer to the Options section for a description of what can cause each of these events. Note that all of the event conditions, except for Execute, halt execution after the event occurs.
• Execute: Halts execution when this location is about to be executed.
• Write: Halts execution after this location has been written to.
• Inc/dec: Halts execution after either field of this location has been incremented or decremented.
• Compare: Halts execution after this location has been used in a comparison
• Read: Halts execution after this location has been read from.
• Test: Halts execution whenever this location is changed in such a way that an expression that you specify evaluates to true. The expression may involve the A and B operands. Note that the test expression is checked only when the contents of the location are modified.
• Frequency – Specifies how many times the event conditions need to occur before execution halts. A frequency of 1 means execution halts after each event. A frequency of 10 means that execution will halt every 10^th time the conditions are met. A frequency of 0 will never halt execution. (This last feature allows breakpoints to be used as a tracking tool, for instance to determine where instructions are being copied from/to.)
• Movability – Breakpoints can be either fixed or movable. A movable breakpoint is copied along with the rest of the instruction by a MOV.I instruction; fixed breakpoints are not.

Breakpoints can be set in source code using four compiler directives:

• ;debug [static | off] – Enables the setting of breakpoints. The “off” keyword disables breakpoints, so that no breakpoints are set until the next ;debug directive. ;debug static causes all further breakpoints to be fixed, rather than movable (the default).
• ;trace [off] – Sets breakpoints at all subsequent instructions, until a ;trace off directive.
• ;condition [EWICRT] [“<condition>”] [M | F] [<frequency>] – Sets the default set of conditions for all subsequent breakpoints. All parameters are optional, and may appear in any order.
• Any combination of the letters E, W, I, C, R, and/or T, in any order, specifies what events will cause a halt. If none of these letters are present, the default event is E (Execute).
• A quoted string sets the expression to be tested if the Test event is specified. Any valid Redcode expression may be used. The variables A (or a) and B (or b) may be used to refer to the values of the A and B operands, respectively. The default is “B==0”.
• M or F specifies a movable or fixed breakpoint, respectively. The default is M.
• Any number present in the conditions sets the frequency. The default is 1.
• ;break [EWICRT] [“<condition>”] [M | F] [<frequency>] – Sets a breakpoint on the next instruction, using the specified conditions. If no conditions are specified, the conditions are those set by the prior ;condition directive (if no ;condition directives have been specified, the default is E M 1).
• Example: ;break WT “A<20” F 10
• This breakpoint will halt execution if the instruction is written to, or if the A operand is less than 20. It is not movable, and halts only every 10^th time.
• The same conditions may also be written this way: ;break fw “a < 20” 10t

Breakpoints can also be set at runtime, from any Core View window, by clicking on the location number to the left of an instruction.

Each unique set of conditions defines a breakpoint type. There are two predefined types: Type 1 halts on Execute, is fixed, and has frequency 1 (E F 1). Type 2 is the same, but movable (E M 1). These two types may be set by a single mouse click in a Core View window. Any breakpoint defined at compile time, or using the Edit Breakpoint window, is given a custom type, greater than 2. The breakpoint type is shown in the Breakpoints dialog box. All breakpoints that are identical (created using the ;trace compile directive, or using the Fill Next option in the Edit Breakpoint window, or copied from a movable breakpoint) share the same type. The total number of different breakpoint types may not exceed 255.

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