More on Picoblaze assembly programming

1. Single bit manipulation





The following code segment shows how to set, clear, and toggle the second LSB of the SO register:





We can also apply the concept of the and mask to the test instruction to check a single bit. For example, the following code segment tests the MSB of the SO register and branches to a proper routine accordingly:



A single bit can be extracted by applying the previous code. For example, the following code segment extracts the MSB of the s0 register and stores it in the sl register:
It is equivalent to "if MSB is zero, don't do anything because s1 is already loaded as 0, if it is not zero (1) then load 0x01 to s1".



2. Multiple-byte manipulation

Assume that x and y are 24-bit data and that each occupies three registers. The following code segment illustrates the use of carry in multiple-byte addition:
The first instruction performs normal addition of the least significant bytes and stores the carry-out bit into the carry flag. The second instruction then includes the carry flag when adding the middle bytes. Similarly, the third instruction uses the carry flag from the previous addition to obtain the result for the most significant bytes.



The incrementing and subtraction of multiple bytes can be achieved in a similar fashion:







Multiple-byte data can be shifted by including the carry flag in the individual shift instruction. For example, the sla instruction shifts data left one position and shifts the carry flag into LSB. The code for shifting a 3-byte data left can be written as







3. Control structure

A high-level programming language usually contains various control constructs to alter the execution sequence. These include the if-then-else, case, and for-loop statements. On the other hand, PicoBlaze provides only simple conditional and unconditional jump instructions. Despite its simplicity, we can use them with a test or compare instruction to implement the high-level control constructs. The following examples illustrate the construction of the if-then-else, case, and for-loop statements. Let us first consider the if-then-else statement:





The code uses the compare instruction to check the sO==sl condition and to set the zero flag. The following jump instruction examines the flag and jumps to the else branch if the flag is not set. The case statement can be considered as a multiway jump, in which execution is transferred according to the value of the selection expression. The following statement uses the SO variable as the selection expression and jumps to the corresponding branch:



The multiway jump can be implemented by a hardware feature known as "index address mode" in some processors. However, since PicoBlaze does not support this feature, the case statement has to be constructed as a sequence of if-then-else statements. In other words, the previous case statement is treated as



The corresponding assembly code segment becomes:



The for-loop statement executes a segment of the code repetitively. The loop statement can be implemented by using a counter to keep track of the iteration number. For example, consider the following:



The assembly code segment is:



4. Subroutine development

PicoBlaze uses the call and return instructions to implement the subroutine. The call instruction saves the current content ofthe program counter and transfers program execution to the starting address of a subroutine. A subroutine ends with a return instruction, which restores the saved program counter and resumes the previous execution.

Because of the primitive nature of the assembly language, thorough documentation is instrumental. A subroutine should include a descriptive header and detailed comments. A representative header is shown below: