IF_x (Conditions)

Every Propeller Assembly instruction has an optional "condition" field that is used to dynamically determine whether or not it executes when it is reached at run time. The basic syntax for Propeller Assembly instructions is:

< Label > < Condition > Instruction Operands < Effects >

The optional Condition field can contain one of 32 conditions (see the Conditions table below) and defaults to IF_ALWAYS when no condition is specified. The 4-bit Value shown for each condition is the value used for the –CON– field in the instruction's opcode.

This feature, along with proper use of instructions' optional Effects field, makes Propeller Assembly very powerful. Flags can be affected at will and later instructions can be conditionally executed based on the results. Here's an example:

         test    _pins,    #$20    wc
         and     _pins,    #$38
         shl     t1,       _pins
         shr     _pins,    #3
         movd    vcfg,     _pins
if_nc    mov     dira,     t1
if_nc    mov     dirb,     #0
if_c     mov     dira,     #0
if_c     mov     dirb,     t1 

The first instruction, test _pins, #$20 wc, performs its operation and adjusts the state of the C flag because the WC effect was specified. The next four instructions perform operations that could affect the C flag, but they do not affect it because no WC effect was specified. This means that the state of the C flag is preserved since it was last modified by the first instruction. The last four instructions are conditionally executed based on the state of the C flag that was set five instructions prior. Among the last four instructions, the first two mov instructions have if_nc conditions, causing them to execute only "if not C" (if C = 0). The last two mov instructions have if_c conditions, causing them to execute only "if C" (if C = 1). In this case, the two pairs of mov instructions are executed in a mutually exclusive fashion.

When an instruction's condition evaluates to FALSE, the instruction dynamically becomes a NOP, elapsing 4 clock cycles but affecting no flags or registers. This makes the timing of multi-decision code very deterministic.