Welcome to the SD-8516 Assembly Language Reference Manual! Inside here you will find detailed information and discussion about programming your new SD-8516.

• Lesson 1: “Memory, Registers and Flags”
• Time: 5 min
• Learn:
  • Registers: A B X Y
  • Flags: N Z C V
  • Memory: Flat memory model

In general there are some things you should know and consider first when learning SD-8516 assembly language programming.

One, it's a flat memory model, which goes from $0 to $03FFFF. That's 4 banks of 64k.

Two, word registers are 16 bits. To access them as high byte or low byte use H and L. For example X is 16 bits, but XH is the high byte, and XL is the low byte. Similarly for every register, such as Y, YL is the low byte and YH is the high byte. For A, AH and AL, and so on.

Third, because memory requires three bytes to address, you can only address bank 0 if you use a 16 bit register. To address upper memory, include a bank byte. Like this: BLX, ELM, or GLD, etc. This means BL+X (BL is the bank byte and X is the address within that bank). Or if you use ELM it EL + M, and so on. Always remember that these are not independent registers, but if you modify EL or M it will modify ELM, and if you modify ELM it will modify EL and M as well.

Finally, the concept of flags. Flags are one bit status registers. Some operations modify flags. For example, if you load a zero, the zero flag will be set. if you ADD two numbers which don't fit in a word, the overflow or carry flags might be set. The flags will be explained more in detail in the lessons teaching the individual instructions and how they manage flags. For now know that there are four primary flags that are set via operations:

• N – Negative. If an operation produces a number that looks like a signed negative number, this flag will be set. Most of the time you can just ignore this.
• C – Carry flag. If you add two numbers and it doesn't fit in a word, the carry will be set and then added in on the next ADD (on an ADD-with carry operation). This makes it easy to chain additions of very large numbers by keeping track of the “carry the one” for you.
• V – Overflow flag. In general if there is more overflow than in carry, this is set
• Z – Zero flag. If an operation produces or sees a zero, this is set.

Why flags?

During decision making, you can use flags to control the program flow. This will be discussed in lesson 5: Program flow.

• Lesson 2: “Load-Store Architecture”
• Time: 5 min
• Learn:
  • Registers: A and B
  • Opcodes: LDA, LDB, STA, STB.
  • Addressing modes: Immediate mode (numbers). Memory mode (memory reference).

Let's dive in to the basic idea behind SD-8516 assembly language programming! If you've ever programmed before, it's similar but different to a high level language. It is similar because there are functions and commands that take operands, and it is different because the functions are very simple building blocks, and there are only a limited number of integer variables that you can use.

The first concept is “Load-Store Architecture”. The SD-8516 uses a load-store architecture. This means that data is read from and written to registers, operated on inside registers, and then written back out to memory. There are no memory to memory operations and registers can only be loaded and saved.

The commands to load and store are LD and ST (load and store) followed by the register and an operand. For example,

     ; * LDA means "load into A,"
     ; * LDB means "load into B".
     
     LDA #56             ; Load the decimal number 56 into the variable A.
     LDA [#56]           ; Load the value in memory location #56 into the variable A.

That's it for load operations. You can load a variable with a number either from memory or from a number directly. You cannot load a variable from another variable. This is invalid:

      LDA B              ; this doesn't work.

Next let's look at store operations. Store operations only write to memory. You can't write to a number, that's impossible, and you can't write to another variable. That would violate the “load-store architecture”. Examples:

      STA [$1000]        ; Store the value in A at memory location $ (hex) 1000.

Hex 1000 in decimal is 4,096. You can use decimal numbers via the '#' prefix or hex numbers with the '$' prefix. If you want to use binary, use 0b00000001 (that's the number 1 in binary).

Now you know how to load and store information from memory to the variables!

• Lesson 3: Operations
• Time: 5 min
• Learn:
  • Registers: C and D
  • Opcodes: ADD, SUB

Now, once you have access to information in the computer's memory, you need to be able to perform operations on that information. Some of the things you can do are: adding, subtracting, multiplying and dividing. Here are some examples of things you can do:

      ADD A, B           ; Adds A and B and stores the result in A.
      ADD B, C           ; Adds B and C and stores the result in B

As you can see, the ADD command has a source register and a destination register. The destination is first and the source is second. So ADD A, D means D will be added to A, and A will hold the result. All of the registers such as A, B, C, D can be used. However by convention we like to use A and B for simple math.

Anyways, you can also do these things:

      SUB A, B           ; Subtract A - B and store the result in A.

• Lesson 4: Advanced Operations
• Time: 5 min
• Learn:
  • 32-bit Register Pairing
  • MUL and DIV

Some processors such as the venerable 6502 (6510, etc.) stop with ADD and SUB, but we have a more advanced 8516, so we can also MUL and DIV. However, MUL and DIV are special operations; observe:

      MUL A, B           ; multiply A and B and store the result in AB.

Storing the result in AB? What's that? The SD-8516 has a special 32 bit extended operation for multiplication. The result is stored in A, but if the result would not fit in a word, the extra information is in B and the overflow flag is set. For example. what is $FFFF times $FFFF? It obviously cannot fit in one word. However, the result ($FFFE0001) does fit into two words. So in this case, A would be $FFFE and B would be $0001. This kind of overflow allows muliplication of larger numbers. Before anyone says “Why not just check overflow”, it's because you can also multiply like this:

      MUL AB, CD         ; Multiply AB by CD and store in ABCD.

Now, there is no way to operate on a 64 bit number (ex. ABCD) however, the result will be stored there, for you to interpret. That's the power of the SD-8516, it can multiply quite nicely! If you wanted, you could extend 64 bit operations via software. It would be slow, but workable. “bigwords”?

      DIV A, B           ; Divide A by B and store the answer in A and the remainder in B

The special properties of DIV allow you to perform modulus for free, or, in a modulus operation you can get the DIV for free. You can also do things like:

      DIV AB, CD         ; Divide AB by CD and store in AB and modulus (remainder) in CD.

You can also divide 32 bit paired registers. The powerful MUL and DIV capabilities of the SD-8516 set it apart from other CPUs of the era.

• Lesson 5: Flow Control (Branching)
• Time: 10 min
• Learn: Assembler Labels, CMP, JZ, RET

Tying everything together, what do you think this program does?

      LDA [$00]
      LDB [$02]
      CMP A, B
      JZ @equal
  
  not_equal:
      LDC $01   ; error code #1
      RET
  
  equal:
      LDC $00   ; no error
      RET

The program loads the word (two bytes) at $00 ($00 and $01) into A, and the word at $02 ($02 and $03) into B. Then it compares them. If they are equal, the zero flag is set. Depending on this we set our return code, which here by convention is C. But it could be anything. We have thus demonstrated the ability to compare registers and make a decision on program contorl flow based on that comparison. This has applications everywhere, from making sure a cursor is within the limits of the screen, to testing if a character is uppercase or lowercase, and many, so many applications that we cannot list them here.

CMP is the fundamental flow control operation. Compare two registers and JZ if equal. Fall-through is the not-equal case. You could also use JNZ instead and fall-through the “is equal” case. Now you know how to control the flow of your programs!

CMP works by doing a simple test:

      CMP A, B           ; We are doing A - B!

Yes that's right, it's doing A - B, but it isn't doing it to store the value in A. It's testing if the result is 0 or not. If the result is zero, it sets the zero flag; ZF = 1. If it's not equal, then it is either ABOVE or BELOW zero. Imagine CMP 5,5 versus CMP 5,10 versus CMP 10,5:

      CMP 5, 5           ;  5 -  5 =  0. Aha, a zero! ZF = 1
      CMP 5, 10          ;  5 - 10 = -5. No zero. ZF = 0
      CMP 10, 5          ; 10 -  5 =  5. No zero. ZF = 0

So because it's equal, it produces a zero. Seeing the zero, the CPU sets the zero flag. Then you can control program flow by JZ (jump-if-zero) and JNZ (jump-if-not-zero).

But there is more! As you see above, there are actually three situations that can occurr. It can be equal, or it can be less than zero, or above zero. You will notice that if A is less than B, the number is negative – or, “less than”. And, if the number in A is greater than B, then A-B produces a positive number, which is “greater than” zero. So it means A is greater than zero! This is why it's called CMP or “compare”. It compares if A is greater than, equal to, or less than B. And, we can test that by looking at the carry flag. The rule is, if you need to “borrow”, you do not set carry.

      CMP 5, 5           ;  5 -  5 =  0.  No borrow --> carry is set:     CF = 1
      CMP 5, 10          ;  5 - 10 = -5. Yes borrow --> carry is NOT set: CF = 0
      CMP 10, 5          ; 10 -  5 =  5.  No borrow --> carry is set:     CF = 1

Therefore, if carry is set, we know that A is less than B.

But wait! There's more!

      CMP 5, 5           ;  5 -  5 =  0. Not negative. N flag not set.
      CMP 5, 10          ;  5 - 10 = -5. Yes negative. N flag set!
      CMP 10, 5          ; 10 -  5 =  5. Not negative. N flag NOT set!

So you can also use the N flag. So here is the situation:

• If ZF=1 then A and B are equal.
• If ZF = 0, then look at CF or NF
  • If CF is set, A is greater than B.
  • If NF is set, A is less than B.

There you go! You can do this now, to branch on each condition:

• JZ @A_equals_B
• JC @A_greater_than_B
• JN @A_less_than_B

This is the foundation of how an IF statement works, or the ternary operator in C.

There are sixteen general purpose registers available for use> Here they are, with a short comment on name and purpose. Of course, since they're general purpose, there is nothing separating one register from another except convention. You can feel free to use this guide, or use them any way you like.

Each 16 bit register (such as A) may be accessed as the byte registers H and L. This means AH is the high byte of A, BH is the high byte of B, etc. while AL is the low byte of A, ZL is the low byte of Z, etc.

The system uses register pairing, which simulates 24 and 32 bit registers, for certain limited operations. The allowed pairings are:

• B: BLA, BLY, BLZ and BLT.
• E: ELA, ELB, ELC, ELD, ELX, ELY, ELZ, ELI, ELJ, ELK, ELT, ELM
• F: FLA, FLB, FLC, FLD, FLX, FLY, FLZ, FLM,
• G: GLA, GLB, GLC, GLD, GLI, GLJ, GLK, GLT.

Suggested use is for pointer only since this requires the CPU to manually combine two registers and is therefore much slower than a usual 16 bit register access.

Examples:

• Source pointer ELM, Destination pointer ELD; if it's in an alternate bank, FLD.
• S/D pointers ELI, ELJ or ELX, ELY, ELZ, or across banks you could use ELX, FLY, BLZ if you insisted on XYZ.
• If you needed a second pair you could use FLA, GLB, FLM, GLD, etc.

Warning: B, X and BLX (for example) are not separate! You cannot store something in BLX, then modify B or X independently from destroying BLX. They are built from B and X dynamically by the CPU.

The convention of BLX is that BL is the high-byte. This opposite from 32 bit mode (see below):

The system can simulate 32 bit operations by combining two registers together however be advised this is very slow as it requires the CPU to simulate operations across multiple registers. These otherwise operate like their 24 bit counterparts.

The only allowed pairs are: AB, CD, XY, IJ, TK, LZ, EF, GM.

WARNING: Modifying G or M will destroy GM, etc. as GM is directly made of G and M.

LDGM $12345678 is equivalent to LDG $5678 and LDM $1234. So AB for example uses B as the high-word. This is opposite the BLX convention which uses BL as the high-byte.

This is because during MUL operations overflow moves into the high byte, otherwise it stays in the original register. EX. MUL A, B moves into A but overflow goes into B.

The SD-8516 follows in the grand tradition of no borrow carry. Here's how to understand it:

1. CMP A, B means we do A-B.
2. Then we apply the rules; NO BORROW = CARRY SET

This is often called “No borrow carry”. or “no carry borrow”. Here are some examples:

The common case is CMP X, MAXCOLS. if MAXCOLS is 80, then if X is 0-79 carry will be clear (because a borrow will be needed). This satisfies “no carry, because, borrow”.

LDA #1          ; A = 1
LDB #2          ; B = 2
CMP A, B        ; Compare 1 with 2
                ; Performs: 1 - 2 = -1 (needs borrow)
                ; 1 >= 2? NO
                ; CARRY = 0 (borrow needed)

In the above example, A is less than 2, therefore a carry (i.e. borrow) will be needed. This is “NO BORROW = CARRY”.

CARRY = 0 because A < B.

1. # The Rule:

``` CMP A, B (performs A - B)

CARRY = 1 if A >= B (no borrow needed) CARRY = 0 if A < B (borrow needed)

Just remember C = A ≥ M. You can also say it as ABC; remember your ABC's: C = A ≥ B or A ≥ B = C. The sign points in the direction you read the letters, i.e. >= so it is easy to remember. “ABC… A>=B –> C.”