• Lessons 0-7: SD-8516 Assembly Language
• Lessons 8-15: This page.

In part 1 we learned some basics about the ISA (instruction set architecture) and the architecture of the CPU. In part II we will wrap up the most important opcodes, but we will also lean into how they are used to get things done.

• Lesson 8: “Special Flags”
• Time: 10 min
• Learn: All Available Flags

In the previous lesson on flags you learned about the Z, N, C and V flags. These are used by the CPU to indicate the status of various operations. For example, the zero flag is used to indicate the last operation produced a zero. Therefore if you are looking for the zero at the end of a string,

      LDC #0    ; zero C (string starts at length 0)
      
  strlen_loop:
      LDAL [ELM]
      JZ @strlen_end
      INC C                ; we found a non-zero character in the string.
      JMP @strlen_loop
      
  strlen_end:
      RET                  ; C now contains the COUNT of all non-zero characters in a string

…you will notice that the JZ works with LOAD instructions (here, LDAL loads one byte). ; if the byte retreived is a zero, it will set the zero flag. You do not need to CMP AL, 0 – it's automatic.

However, there are other flags; The first four user-facing flags are E, F, B and U. You can set these flags and unset them in the same way as Z N C V – ex. setting ZNCV is done with SEZ, SEN, SEC and SEV; unsetting them is done with CLZ, CLN, CLC and CLV. The E F B U flags are set and unset with:

• SEE and CLE for the E (extended, or 'extra') flag.
• SEF and SEB for the F flag (or 'flag' flag).
• SEB and SEU, CLB and CLU for the B (bonus) and U (user) flags.

On a technical level the E flag is reserved as it is used to deal with BCD; but since we deprecated BCD instructions it is currently an unused flag. In any case, the F, B and U flags are never set by the CPU and may be used by user functions. A common use is to return a 1 bit status; 0 for no error and set (1) for error. Since these flags are never set by the CPU they are easy to control. Using the Z or C flags is dangerous since some instructions may corrupt those flags.

Your programs can also use them as 1 bit status variables.

next, the D flag, or debug flag. When set, it will dump instruction data to the javascript console. This significantly slows down the machine; in fact just having the instructions inline slows down the machine so debug is often removed and ignored in a production or release distribution of the SD-8516. Therefore, for all intents and purposes, you can use SED and CLD as a user flag, just be aware it does affect performance in debug releases.

The I flag (interrupt enable) prevents INT from being called, and is reserved for system use. Not sure what I want to do with it.

The S flag is almost useless; it was intended to turn off a memory trap in the sound system; I found it to be completely useless, maybe a 2% speedup or penalty. it is essentially a user facing flag.

The only flags that you cannot access are the TR (trace), BR (breakpoint) and PR (protected mode) flags. They are so named after the first two letters of their name; but interestingly enough you might as well consider the R to mean restricted. You can't usually set these flags. They are reserved for system use.

  // Arithmetic & User Flags (low byte 0-7)
  Z = 0,   // Zero
  N = 1,   // Negative
  C = 2,   // Carry
  V = 3,   // Overflow
  E = 4,   // Extended carry -- not used/reserved
  F = 5,   // Fast Flags mode. When on, flags are not implicitly checked.
  B = 6,   // BCD/"Bonus" flag. Have fun!
  U = 7,   // User flag. For users to use.

  // Control & Operation Flags (high byte 8-15)
  D = 8,   // Debug mode
  TR = 9,   // Trace mode
  BR = 10,  // Breakpoint mode
  ER = 11,  // Error/Exception (i.e. return code 0 = ok, 1 = error) 'SER' -- set err
  PR = 12,  // Protected mode
  I = 13,  // Interrupt enable
  S = 14   // Sound auto-updates

The key of this lesson is merely to be aware of the flags and the instructions used to set and unset them. In general, they follow the pattern of SEZ and CLZ;; SE(T) and CL(EAR) with the flag letter replacing the parentheses.

Oh, there's one more thing. If you use flags like F, B or U you may notice there is no JF or JNF (jump if F set and jump if F not set). That's because we don't want to add 50 different opcodes to deal with all the flags. What you can do is this:

      ; Some operation that sets the F flag
      TESTF 0x20
      JZ              ; Jump if F is set
      JNZ             ; Jump if F is not set

TESTF works by setting the Z flag if all the bits set in the parameter are also set in the FLAGS register. if you give it a byte it only tests against the bottom 8 bits.

Here's a chart of the bit values for each flag:

  Z = 0x0001 as u16,   // Bit 0
  N = 0x0002 as u16,   // Bit 1
  C = 0x0004 as u16,   // Bit 2
  V = 0x0008 as u16,   // Bit 3
  E = 0x0010 as u16,   // Bit 4 (was X - Extended carry) -- SEE and CLE can be used as a user-flag (is never set by an opcode)
  F = 0x0020 as u16,   // Bit 5 (Fast/deprecated) -- SEF and CLF can be used as a user-flag (is never set by an opcode)
  B = 0x0040 as u16,   // Bit 6 (Bonus/BCD) -- SEB and CLB can be used as a user-flag (is never set by an opcode)
  U = 0x0080 as u16,   // Bit 7 (User flag) -- SEU and CLU can be used as a user-flag (is never set by an opcode)
  D = 0x0100 as u16,   // Bit 8 (Debug)
  TR = 0x0200 as u16,  // Bit 9 (Trace)
  BR = 0x0400 as u16,  // Bit 10 (Breakpoint)
  ER = 0x0800 as u16,  // Bit 11 (Error/Exception)
  PR = 0x1000 as u16,  // Bit 12 (Protected Mode)
  I = 0x2000 as u16,   // Bit 13 (Interrupt)
  S = 0x4000 as u16    // Bit 14 (Sound)

The stack is a concept held over from the early days when there were very few instructions available. If you consider a minimal ISA, you need instructions to load and store from memory, an instruction to compare, and so forth. In such a minimal architecture, loading and storing from memory has certain emergent properties. For example if you have a list of things, their position in memory is not random because you are incrementing a counter such as a memory pointer to traverse that list. It is this way of doing things that we remember when we use the stack.

The stack is just a data structure. But it is so important and fundamental that is baked into the instruction set of the CPU. This is a common theme; important things that people found they needed to do all the time became instructions. Even in a minimal-instruction set design (MISC) or reduced instruction set design (RISC) you will find instructions like PUSH and POP because they are some of the first things that were turned into instructions after fundamental operations like LOAD, STORE, AND and ADD.

The stack is an area of memory that you can PUSH and POP values to, in order. For example, you can PUSH the number 5 and the number 5 will be “on top” of the stack. Then you can “POP” it later. The stack is like an array but you can only go forwards and backwards, and reading the stack destroys it. This is a lot like how old magnetic ring memory worked, in a way.

Today, we would call the stack a LIFO buffer; a “Last-in, First-out” data structure. If I do this:

  PUSH 1
  PUSH 6
  PUSH 5

then three successive POPs will return 5, then 6, then 1 – the reverse of the order you PUSH'ed them.

When you CALL or JSR (jump to subroutine) to function, the CPU pushes the return address onto the stack. Then a subsequent RET or RTS (return from subroutine) will POP the return address back into IP (instruction pointer) or PC (program counter) so that the next instruction loaded will be after the original CALL.

There are many uses for the stack but the most common is to temporarily save values. If you understand that you are 90% of the way there!

For interrupts, it also pushes the registers and flags. You can do this manually if you want to save the registers on a function call. For example if you call a function with a pointer to a string, you might modify that pointer to find the end of the string (looking for a zero). That's what a strlen function does. So you PUSH the pointer register at the start and POP it after, to “save” the register back to where it was when the function was called. This way the code that calls strlen can then call strcpy without having to replace the string pointer.

Another use is for IL (intermediate languages). They use RPN (Reverse Polish Notation) to store any kind of math equation on the stack.

An ADD function will do this: One, the interpreter will push the two numbers and then push the add command. Then an interpreter will POP the add function, and then it knows to POP two numbers, add them, and push the result back on the stack. Why? to make ADD independent. Like a dispatcher for a mini CPU. Next, whatever function comes next just POPS the result off the stack. So you can print it, assign it to a variable, or use the result as part of a larger operation. For example, how do you interpet 5 * 2 - 1 + 6? Simple. You push 5 2 * 1 - 6 + and the computer will push and pop the results, like a mini CPU of its own.

Pop + tells it to pop two numbers and add them. The first number is 6. The second is a minus. Minus what? it pops two things, a 1 and a *. Multiply what? Multiply pops 5 and 2, multiplies them, and pushes 10 on the stack. This is the popped by the minus, which subtracts 1 from 10, pushing a 9. This then goes back to the + which adds the 9 and the 6 to get 15. This is how recursion and RPN is used to represent any equation on a stack.

The last one we will discuss is function calls from a higher level language. Often times when you compile a language like C it will put local variables on the stack. Then when you return from that function they all get popped. While they are on the stack they are accessed like [SP+index] so int c=5 would be:

  STA [SP+1], 5

And when that local space is no longer needed it is POP'ed into a register which is then restored, via POP, at the end of the function. This method of keeping data on the stack is called a stack frame. Compilers like to use stack frames because they don't always know how many registers a CPU has and they need to work on different CPUs, like how GCC or LLVM works on windows, mac, amd, and many others.

Understanding the stack is not too hard, but it's important! So, that's about it for this lesson.