SC/MP Introduction

        .cr     scmp        To load this cross overlay

Programming Model

The programming model in the picture below shows the most important registers of the SC/MP processor. I only include a little summary about the features of the SC/MP's programming model here. It is not my intention to make the original documentation obsolete, so please refer to the original documentation for further details.

The Accumulator

The Accumulator is the most important register for 8 bit arithmetic operations. Its standard name is AC, which is a reserved word.

The Extension Register

The Extension Register E is usually used to hold the second operand for instructions which require 2 operands. In which case the Accumulator holds the other operand.
If the displacement value for indexed or auto-indexed memory instructions equals -128 the contents of E is used as displacement value.
The Extension Register is also used as serial input or output shift register.

The Status Register

The SR contains 3 system flags and 5 I/O flags:

Bit 7	CY	The Carry/Link flag
Bit 6	OV	Overflow Flag
Bit 5	SB	Single bit input
Bit 4	SA	Single bit input / interrupt input
Bit 3	IE	Interrupt enabled if 1
Bit 2	F3	Single bit output
Bit 1	F2	Single bit output
Bit 0	F1	Single bit output

Please note that the SC/MP doesn't have a Zero or Negative flag. These conditions are directly tested on the contents of the Accumulator.

PC, Primary Program Counter

This 16 bit register is the program counter. It can either be named PC or P0, which are both reserved words.
Although this register is 16 bits long, it will access memory in pages of 4kB in size. When the program counter reaches the end of a page, it will wrap around to the beginning of that same page. You'll have to jump to another page to get out of the current page.
PC relative instructions are confined in the same memory page. Displacements which would cross a page boundary simply wrap back. The assembler will warn you if a displacement crosses a page boundary.
Two byte instructions which cross the page boundary will cause an error message by the assembler.

The SC/MP has a rather peculiar way in which it fetches instructions. It increments the program counter before fetching the instruction. Therefore you can't use the first address of each 4kB memory page for instructions.
It also affects the way you have to jump to other locations in memory. You'll have to manually subtract 1 from the address to which you want to jump.

Pointer Registers

There are three 16 bit pointer registers which can be used as memory or peripheral addressing, page pointers, stack pointers or index registers. Typically these registers are dedicated to the following tasks, although you're free to use any register as you please.

P1 - ROM pointer
P2 - Stack pointer
P3 - Subroutine pointer

You can load each half of all three pointers independently. Or you can swap the contents from one of the pointers with the program counter. Swapping one of the pointers with the PC register effectively is a one level subroutine call. Swapping the registers back again is like a return from subroutine.
If you need deeper nesting of subroutines you'll have to implement a software stack and push the return address to that stack manually.

Timing

SB-Assembler Version 3 can show you the cycle times of each instruction when the TON list flag is switched on. The numbers presented are the number of clock pulses needed to execute the instruction.
For conditional instructions 2 times are given. The lowest values are when the condition is false, while the highest times are for when the condition is true.

Reserved Words

The SB-Assembler SC/MP Cross Overlay has a few reserved words. Reserved words are all register names. You better avoid these reserved words when you assign your own labels. E.g. don't call your labels AC, E, PC.
If you do use the reserved words as label names you may expect unpredictable behaviour of the assembler sooner or later. Please note that the assembler will not warn you if you try to assign a label with a reserved name!
Reserved names can not be used in expressions, like label names can. An Undefined label error will be reported if you do try to use a reserved word in an expression because it is treated as a normal label in this case.

Here's the list of all reserved words:

AC, E, PC, P0, P1, P2, P3, SR

Special Features

New addressing mode and assumptions

As already mentioned an offset of -128 will use the contents of the E register as offset. With the SB-Assembler you can also use E instead of an offset of -128 if you like.

If you don't specify an offset an offset of 0 is assumed.

Absolute address is assumed for PC relative instructions where the (PC) pointer is omitted.

Normally the index registers are named P0, P1, P2 and P3. PC is an alias for P0 and may be used interchangeably. As of version 3.01.02 of the SC/MP cross overlay it also allowed to use only the digits 0 to 3 to reference the index registers.

Extra Instructions

The SC/MP is so simple that it doesn't even know subroutines. You can simulate a one level subroutine call by swapping a pointer register with the program counter. Swapping the pointer register again will simulate the return from subroutine. Per convention pointer register P3 is used for this swapping purpose. However the SB-Assembler allows you to use any of the 3 pointers, giving you a total of 3 separate one level subroutines.
I have added 2 instructions which will help simulate a one level subroutine system.

JS   Jump to Subroutine

Syntax

        JS  pntr,destination

Description

This instruction can be used to call to this one level subroutine. It simply transfers control to the destination, using one of the 3 pointer registers P1 to P3.
The SB-Assembler translates this instruction into the following set of instructions:

        LDI     /destination-1
        XPAH    pntr
        LDI     #destination-1
        XPAL    pntr
        XPPC    pntr

RET   Return from Subroutine

Syntax

        RET     pntr

Description

The RET instruction is simply an alias for the XPPC instruction. It is included to make its function more obvious. So the RET instruction swaps the pntr with the program counter, giving control back to the code which called the subroutine.

Weird and wonderful things

Due to the limited addressing capability of the SC/MP processor itself and the rather primitive original SC/MP assembler, programmers were used to jump through all kinds of hoops in order to get their work done. The SB-Assembler is far more advanced than the original SC/MP assembler. Therefore I had to make some compromises.
I became aware of the quirks of the SC/MP assembler while adapting the Elbug monitor (Elektor SC/MP monitor) for the SB-Assembler. I had a list file and a hex file of this monitor, and I thought it would be a good test case for the SB-Assembler. If the SB-Assembler can produce the same code, I'm all set.

First of all the original SC/MP assembler had only limited calculation capabilities. As you can see from the instruction set the E register is used as index when the offset is -128. The Elbug source had a few lines which used a constant offset of $80 in stead of -128. On a byte level this may be correct. But the SB-Assembler rightfully disagrees here.
An offset of $80 is too high, it's 128, whereas the maximum positive offset is only 127. I could make an exception for $80, but that would mean that the assembler would not be able to raise an error if I accidentally entered an out of range offset any more.
So I had to make small adaptions to the source code of the Elbug monitor, changing the $80 offset to -128. B.T.W., I could also have used E as offset, to indicate the use of the E register.

Addressing RAM can be done with one of the three index registers P1 to P3. The programmer of the Elbug monitor decided to use the PC registers instead in some situations. I must say it's clever programming, but it does obscure things a lot, especially for beginners. And I thought the S in SC/MP stood for Simple.
To cut a long story short the Elbug source deliberately uses the limitations of the original SC/MP assembler to calculate the offset from the current PC value to RAM locations. How is this possible? The program starts at address $0001, whereas the monitor RAM addresses are located from $FE0 to $FFF, which is at the end of the current page. By using a negative offset to the PC value, the effective address wraps back in the current page, ending up at addresses somewhere near the monitor RAM locations.
Please note that this trick is only possible right at the beginning of a page, where the RAM locations are placed right at the end of that page. It is also possible to wrap from the end of the page, back to the beginning of that page. As of SC/MP Cross overlay 3.02.00 the SB-Assembler doesn't issue a range error any more when such a page wrapping occurs.

Error Messages

The SC/MP Cross Overlay adds 1 extra error message and 2 extra warning messages to the standard list of possible errors/warnings.

Instruction crossed a page boundary

This is an error condition. It is raised when the operand part of an instruction is written to a different memory page than its opcode part. The processor is unable to get both, because the program counter wraps back to the beginning of the current page before it can get the operand.
Move your program away from the page end. Perhaps you should split your program into smaller pieces, which each fit into its own page.

Instruction starts at page boundary

This is a warning. An instruction at the first byte of a memory page can not be executed. But you may have done this deliberately. Therefore this is only a warning.
You may wish to ignore it. But you may also wish to move your program one byte up in memory, to avoid this warning.

Offset is -128, E register conflict

This is also a warning. You have provided an absolute address, which was translated into an offset of -128. Offsets of -128 are normally replaced by the value of the E register.
So you may want to correct this, otherwise you may get unexpected results.

Overlay Initialization

• The SC/MP processor doesn't have any instructions which use a 16-bit operand. So I have no way of knowing what endian model I should use. Therefore I have decided to use the little endian mode for the SC/MP Cross Overlay.
• The current memory page is set to 0.
• The maximum program counter is set to $FFFF

Differences Between Other Assemblers

There are some differences between the SB-Assembler and other assemblers for the SC/MP processor. These differences require you to adapt existing source files before they can be assembled by the SB-Assembler. This is not too difficult though, and is the (small) price you have to pay for having a very universal cross assembler.

• The SC/MP instruction set knows special mnemonics that indicate immediate addressing mode. Therefore it is not absolutely necessary to add the immediate mode identifier # in front of the operand. Although it wouldn't hurt if you did.
  E.g. the two instructions LDI VALUE and ldI #VALUE have exactly the same effect.
  If you want another part of the 32-bit number to be loaded immediately you could use the other immediate mode identifiers /, =, or \.
• The obvious differences in notation of directives common to all SB-Assembler crosses.
• Don't forget that the SB-Assembler does not allow spaces in or between operands. Only Version 3 will allow one space after each comma separating operands in the operand field.

Special thanks

Special thanks to Graham (SiriusHardware) from the UK Vintage Radio Repair and Restoration forum, who corrected a bug in the displacement calculation for jump instructions. I couldn’t get that right due to my lack of experience with the SC/MP processor. They’ve reached out to me with a corrected version of this overlay, which is now included in the download file and on my github.