Bit-Slice Design: Controllers and ALUs

Simple Controller continued

Last Edit October 10, 1996; May 1, 1999; July 9, 2001

Conditional Branch

It is sometimes desirable to terminate a microroutine in one of several different ways depending on one or more conditions. For example, the conditions tested could be the various status bit outputs of the ALU based on the result of an operation. Status outputs can include

1. Z=1 if ACC=0
2. S=1 if ACC < 0
3. C_out=1 if | ACC | > range
4. OVR (overflow)=1 if error
5. on the result of a compare operation

Figure 2-12 Conditional Branch Flow Diagram. "IF-THEN-ELSE" - the conditional branch (CJP, conditional jump)

The ability to test a condition and to branch if the condition is TRUE is provided by adding a MUX at the counter load control. The load control filed in the microword is changed to a branch condition select, which selects

1. ground (count; no branch)
2. condition 1 or condition 2 (load branch address into counter if TRUE)
3. Vcc (unconditional branch, load counter).

Figure 2-13 CCU with Conditional Branch Capability

Figure 2-14 Microword Format for Conditional Branch (A) Detail of Load Control MUX (B) Detail of Microword Format

A sample piece of microcode is shown in Figure 2-15 that has both unconditional and conditional branches. As with most programming languages, if a conditional test fails (FALSE), execution continues sequentially, as shown in statements at addresses 30 and 31. The branch condition select field is 2 bits wide in this example. A wider field would allow a larger MUX and therefore would allow more conditions to be tested.

Figure 2-15 Microcode, Demonstrating Conditional Jumps