15 KiB
SplitBit Assembler Manual:
SplitBit assembly syntax is similar to many other assembler syntaxes. Whitespace at the start or end of a line is disregarded by the assembler and may be used to make programs more readable to the programmer. The Instruction Mnemonics are listed in the SplitBit Programming Manual, and the assembler is not case sensitive in regard to the mnemonics.
A semicolon, ';', denotes the start of a comment, anything beyond it on a line is disregarded by the assembler.
Special Keywords are denoted with hash marks, '#'. The Keywords are #Include, #Program, #Data, #Vectors, #Align, and #Reserve.
The first four say what kind of thing follows them. #Align and #Reserve are instructions to the assembler in the middle of a segment, and are described under Moving The Cursor Along.
SplitBit programs must have a Program Segment. You define the start of a program with the #Program Keyword. SplitBit programs may have a Data Segment. You may define the start of the data with the #Data Keyword. SplitBit programs may have a Vector Segment. You define it with the #Vectors Keyword. See The Vector Segment below.
Literal Values:
Literal values may be defined in a few ways. Numerical values must be within the range of a single 8 bit integer. The assembler will accept:
- Hexadecimal values prefaced with 0x, eg. 0x00, 0x7F.
- Decimal values prefaced with 0d, eg. 0d0, 0d120, 0d255.
- Strings enclosed in double quotes, eg. "a", "Hello, World!", "It is dark, you are likely to be eaten by a grue."
Any token beginning with a '0' is read as a numerical literal, so a malformed one is an error rather than something the assembler tries to interpret as a label. This also means a label cannot begin with a '0'.
Labels:
Labels may be a string of up to 32 alphanumeric characters that must end with a colon, ':'.
programStart:
loopStart:
errorHandler01:
A name may only be defined once across a program and everything it includes. Defining it twice is an error, because otherwise a reference resolves to whichever definition came first, and a typo or a name that two libraries both happen to use is very hard to track down.
A label may be referenced by name, without the colon, to place its two byte address wherever the reference appears.
In the Program Segment that is how the branch instructions and SETD are given somewhere to go. In the Data Segment it writes the address down as data, which is how a table of addresses is built for LDD to walk.
#Data
One:
"one"
Two:
"two"
Table: ; Two entries, each the two byte address of a string above.
One
Two
Naming a Data Pointer:
The instructions that work through a Data Pointer name which one by hanging a selector off the mnemonic, after a full stop.
LDA.2 ; Load A through Data Pointer 2.
STA.1 ; Store A through Data Pointer 1.
INCD.2 ; Step Data Pointer 2 along.
SETD.3 Grid ; Aim Data Pointer 3 at Grid.
Leave the selector off and the instruction uses Data Pointer 0, so a program that only needs one pointer never has to write one.
LDA ; Exactly the same as LDA.0
LDD and STD move a pointer through a pointer, so they take two selectors. The first names the pointer being moved and the second names the pointer that addresses it. Either may be left off, and again means Data Pointer 0.
LDD.1.0 ; Data Pointer 1 becomes the address stored at Data Pointer 0.
STD.1.0 ; Store Data Pointer 1 into the memory addressed by Data Pointer 0.
LDD.2 ; Same as LDD.2.0
LDD ; Same as LDD.0.0, which makes DP0 follow the address it holds.
Writing a selector on an instruction that does not work through a Data Pointer is an error, as is naming a pointer the machine does not have, or giving an instruction more selectors than it takes.
Instruction Operands:
Instructions that read operand bytes out of Program Memory must be followed by those operands. The branch instructions and CALL take a label; SETD takes a label or a pair of literal bytes; INIA, INIB, DPUP, DPDN, and the input and output instructions each take a single literal byte; SWI takes the name of a vector, or a literal number.
Leaving an operand off is an error rather than something the assembler works around, because the instruction would otherwise take whatever followed it as the operand and every address after that would shift.
Data Pointer selectors do not count as operands here, because they are written on the mnemonic rather than after it.
Moving The Cursor Along:
Both segments are written from the beginning, and every label stands for wherever the cursor had reached when the assembler met it. Two directives move that cursor without you having to write zeroes by hand.
#Align puts down as many zero bytes as it takes to reach the next multiple of the number that follows it.
#Data
#Align 0x100
Segment: ; Guaranteed to begin at a page boundary.
This matters for code that does address arithmetic on a pointer's low byte and treats the carry out as reaching the end of something. Both prime sieves work that way, and both now ask for the boundary themselves. Before this existed they relied on print.asm padding its data out to a whole page, which worked but put the requirement in a different file from the code that needed it, and quietly charged every other program 253 bytes for it.
#Reserve puts down the number of zero bytes that follows it, so that a label can stand for a whole region rather than just its first byte.
#Data
Buffer:
#Reserve 0d256 ; Anything after this begins 256 bytes further on.
Next:
Without it a label like Buffer is one byte as far as the assembler knows, so a later label lands inside the region and the two quietly overlap.
Both take a number written the way literals are, prefaced with 0x or 0d, but the number may go up to 0xFFFF rather than being held to a single byte. Neither number is ever emitted, so a byte's range would be the wrong limit: a page alignment needs 256, and a reservation is often much larger.
Both work in the Program Segment as well as the Data Segment, and both are an error anywhere else, because outside a segment there is no cursor to move.
The Vector Segment:
A vector says where to go when something happens: the machine starting up, a program asking for a service, a device wanting attention, or the CPU meeting a byte it cannot decode. The Vector Segment says which of your routines belongs to which vector, and the assembler works out the rest.
A program does not need one. Without a Vector Segment a program starts at the beginning and behaves exactly as it always has.
Every line names a vector and then the label of the routine that handles it.
#Vectors
Boot realStart
BadOpcode reportFault
openFile openFileHandler
Device 0x10 diskReady
Three names already mean something:
| Name | Vector |
|---|---|
| Boot | Where the machine begins at power on. Without this a program starts at the beginning of its Program Segment. |
| SoftReset | A warm restart. SWI SoftReset is how a program asks for one. |
| BadOpcode | The CPU met a byte that is not an instruction. |
Anything else you name is a software interrupt of your own. You do not choose its number and you never write one: the assembler allocates them in the order they appear, starting above the range held back for faults that do not exist yet. That is the same bargain as labels everywhere else in SplitBit assembly, where you name a thing and let the assembler work out where it went.
You then use the name as the operand of SWI:
SWI openFile
A device is different, because its number is not a choice. A device interrupts on the port it is plugged into, so the Device line says which port rather than giving it a name of its own. The port is a literal value, and the routine after it handles that device.
The assembler will refuse two handlers for the same vector, a name used with SWI that no Vector Segment gives a handler to, and a handler that is not a label.
Vectors 3 through 15 are held back for faults that have not been defined yet. They have no names, so there is currently no way to write a handler for one, and none is needed: each will be given a name of its own as the fault it stands for is defined. Because you never write a vector number, there is no way to land on one of them by accident either.
Including Other Files:
The #Include Keyword tells the assembler to load another file to be assembled along with the current file. It is more or less equivalent to copying the contents of the included file into the current file being processed. You simply put the name of the file to include after the keyword.
#Include print.asm
The assembler looks for that file in two places, in this order:
- Beside the file that asked for it. A library including its own siblings needs no help.
- Along the include directories given with -I on the command line, in the order they were given.
An absolute path is taken as it is written. If the file turns up nowhere, the assembler says so and lists every place it looked.
Because a library is normally referred to by name alone, a program that uses one has to be told where the libraries live:
Assembler -I Libraries primeSieve/8bitSieve.asm
Including the same file twice does nothing the second time, so two libraries may both depend on a third without the program that uses them having to know. The assembler compares files by where they really are rather than by how they were spelled, so the same library reached by two different routes is still only assembled once.
Running the Assembler:
Assembler [options] <sourcefile>
| Option | Meaning |
|---|---|
| -o, --output <file> | Write the binary to this path. Without it, the binary is named after the source file, with a .bin extension, in the directory the assembler was run from. |
| -I, --include <dir> | Look in this directory for included files. May be given more than once, and the directories are searched in the order given. |
| -M, --depend <file> | Write out which source files went into the binary, as a make rule. |
| -h, --help | Print the options and stop. |
The assembler stops at the first error, says which file and line it was in, and exits without writing a binary.
Building With Make:
The -o and -M options are there so that the assembler fits into a build system. -o puts the binary wherever the build wants it, and -M writes down which libraries went into it, so that editing a library reassembles every program that includes it.
$(BUILD)/%.bin: %.asm
@mkdir -p $(@D)
$(ASM) -I Libraries -M $(@:.bin=.d) -o $@ $<
-include $(BINARIES:.bin=.d)
Programs/makefile in this repository builds every program that way, if you would like a longer example to copy.
An Example SplitBit Assembly Program:
; This is a slightly more advanced hello world program that demonstrates some SplitBit programming conventions.
#Program
start: ; By unenforced convention, Program Labels start with a lowercase letter.
SETD HelloString ; Set the Data Pointer to the address of the string.
CALL printString ; Call the string printing subroutine.
HALT ; End the program.
; This is a reusable subroutine that could be included in other programs.
printString: ; Expects Data Pointer to be set to the beginning of the string to be printed.
LDA ; Move the first character of the string into A.
BRA printDone ; If A is NULL, the string is finished, so return.
OUTA 0x00 ; Output the character.
INCD ; Increment Data Pointer to the next character.
BRI printString ; Branch to the beginning of the loop.
printDone:
RET ; Return to the caller.
#Data
HelloString: ; By unenforced convention, Data Labels start with a capital letter.
"Hello, World!"
An Example Using More Than One Data Pointer:
Copying between two places in Data Memory needs two pointers: one to read through and one to write through. With a single pointer this loop has to save and restore it on every pass.
; Copy a string from one place in Data Memory to another.
#Program
start:
SETD.0 Source ; DP0 walks the source.
SETD.1 Dest ; DP1 walks the destination.
copy:
LDA.0 ; Read a byte through DP0.
BRA copyDone ; A zero byte is the end of the string.
STA.1 ; Write it through DP1.
INCD.0
INCD.1
BRI copy
copyDone:
HALT
#Data
Source:
"Copied through two pointers."
Dest:
0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00
0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00
Remember that DP0, DP1 and DP2 survive a CALL, so a loop like this one can call a subroutine in the middle without losing either pointer. DP3 does not survive, which is what makes it the pointer a subroutine uses to hand an address back.
An Example Using Interrupts:
This program installs three handlers and never writes a vector number. The Boot Vector sends the machine somewhere other than the first byte of the program, a trap the program names for itself is reached with SWI, and the test device on port 0x10 is caught when it asks for attention.
; Interrupt handling from all three directions.
#Program
start:
CIF ; Hold devices off while we set up.
SETD.0 Greeting
CALL printString
SWI announce ; A trap of our own, reached by name.
INIA 0d1
OUTA 0x10 ; Ask the test device for attention. Its line goes up.
SIF ; Let it through. It is answered before the next instruction.
HALT
; A trap. It is entered with a full frame, so it may use any register it likes
; without agreeing anything with the code it interrupted.
announce:
SETD.0 Trapped
CALL printString
RETI
; The device handler. Reached because the device sits on port 0x10.
deviceReady:
SETD.0 Device
CALL printString
RETI
; The fault handler. It reports and stops, rather than trying to carry on.
reportFault:
SETD.0 Broken
CALL printString
HALT
printString: ; Expects DP0 to be set to the beginning of the string.
LDA.0
BRA printDone
OUTA 0x00
INCD.0
BRI printString
printDone:
INIA 0x0A
OUTA 0x00
RET
#Data
Greeting:
"ready"
Trapped:
"trap"
Device:
"device"
Broken:
"bad opcode"
#Vectors
Boot start ; Begin here rather than at the first byte.
BadOpcode reportFault
announce announce ; A name of our own. The assembler numbers it.
Device 0x10 deviceReady ; Named by the port, because that is what decides it.
The output is ready, trap, then device.
Note that a vector name and a routine name are kept apart, so naming both announce is allowed. If that reads as confusing, name them differently: nothing requires them to match.