Parameter passing: PABs
Foreign language translators
XML >19: the card ROM scanner
XML >1A: the GROM scanner
Calling XMLs: the crude way
Calling GPL and XML: the smooth way
String allocation routine
Passing parameters from Basic
Returning values to Basic
Standard headers can be located either in peripheral cards ROM, in GROMs or in cartridge ROM. In peripheral cards, the header always appear at address >4000. In cartridge ROM it maps at >6000 and can only contain programs. In GROMs there can be a header at the following addresses: >0000, >2000, >4000, >6000, >8000, >A000, >C000, and >E000 in GROM memory. If a header is present at any of these addresses, the first byte must be >AA to to tell the scanning routines there is a header here.
Headers contains lists of programs, subprograms, power-up routines, DSRs and ISRs. Lists are organised as chains of linked items: each item begins with a word pointing to the next item in the list. The word is >0000 for the last item in the list.
Power-up routines are executed when you reset the TI-99/4A, before the user can press a key to leave the main title screen. They can be found in GROMs or peripheral card ROMs, but not in cartridge ROM.
Programs are what appear on the main menu, after the user left the title screen: "Press 1 for TI Basic", etc. Programs can be located in GROMs or in cartridge ROMs, but not in peripheral cards. At some point in time, Texas Instruments modified the system so that programs in ROM won't appear on the main menu: according to which version you have, your console may or may not pick them up.
Subprograms are routines that provide a service to the user. For instance, all Basic CALLs are subprograms. But subprograms can also be called from assembly languages. They can be found in GROMs or peripheral card ROMs, but not in cartridge ROM.
DSRs (Device Service Routines) are special subprograms that deal with files. In Basic, they are called by file operations, and can also be called from assembly. They can be found in GROMs or peripheral card ROMs, but not in cartridge ROM.
ISRs (Interrupt Service Routines) are called when an interrupt was received that did not come from the video-processor. They can only be located in peripheral card ROMs.
Finally, the GROM header at >6000 in base >9800 can have two foreign language translators, used to translate the title screen and the main menu, respectively.
Peripheral cards ROMs cannot contain programs, nor translators.
GROMs cannot contain ISRs.
Cartridge ROMs can only contain programs.
|>x000||>AA indicates a standard header|
|>0xx2||Number of programs (optional)|
|>x004||Pointer to power-up list (>0000 if none)|
|>x006||Pointer to program list (>0000 if none)|
|>x008||Pointer the DSR list (>0000 if none)|
|>x00A||Pointer to subprogram list (>0000 if none)|
Program, subprogram and DSR lists:
Link to next item --+
Name length |
Link to next item: >0000
Power-up and ISR lists:
Link to next item --+
Link to next item: >0000
>4000: >AA Standard header
>4010: >0000 Link to next power-up (no more)
>4018: >4022 Link to next subprogram
>4030: >403A Link to next DSR
>4050: >0000 Link to next ISR (no more)
>4100: ... Power-up routine
>4200: ... SUB1 subprogram
>4220: ... >B3 subprogram / DSR
>4300: ... MYDSR and DUMMY DSRs
>4440: ... >04 DSR
>4800 ... ISR
As the name indicates, a power-up routine is executed at power-up time, i.e. just after the TI copyright screen has been displayed, before checking if the user has pressed a key. These routines are meant to initialize the card: the one on the interface card sets the PIO port as output, the one on the disk controller reserves room in the VDP memory, etc. They could also be used to take over control of the TI and prevent the user to go to the main menu. Some third party disk controllers have this annoying habit.
The main power-up program is located in the console GROMs, at address >0020. It performs the following actions:
The power-up searching routine uses XML >19 to scan ROMs and XML >1A to scan GROMs. It branches to each power up routine it finds, whether in assembly or in GPL. Routines are called in the following order:
Assembly language power-up routines should return with B *R11 in assembly.
It is also possible to return with INCT R11, B *R11 but this will prevent further scanning of peripheral cards for power-up routines. It will not however prevent GPL power-up routines from being called.
Things are a little more complicated in GPL as the main program does not branch to power-up routines directly. Instead the addresses of all power-up routine located are saved on the data stack (whose base is not at its usual location, but at >8300, to allow for more storage space). To branch to the next power-up routine on the list, one must therefore transfer its address to the subroutine stack:
DONE DST *>8372,*>8373 Transfer address from data stack to sub stack
If you wanted to shortcut the process and prevent the main program from calling other power-up routines, you should keep searching the data stack until you find an address that is in GROM 0 (i.e. with an address smaller than >1800). This will be the address at which the main routine continues (normally >019F, but it may change according to the GROM version), just branch to it as described above.
DCLR >83D0 This must be done to prevent scanning from resuming
A program is what appears on the main menu, after you left the TI copyright screen: "Press 1 for TI-Basic, 2 for Whatever".
There is no limit to the length of a program name, except that it should fit on one screen line, if you want the main menu to look nice. This being said, nothing prevents you to call your program "PROTECTING YOUR WORLD FROM THE ATTACK OF THE GOULISH DEMONS OF THE LEGIONS OF CHAOS" (except good taste, may be...). However, if another program is found after yours, its name will overwrite the third line of your program name.
Note that at this point only the standard character set has been loaded in VDP memory (unless you loaded another one with a power-up routine). Therefore you can only use characters 32 to 96 in your program name, i.e. no lower case.
The main power-up program lists the programs it finds in the following order:
Once the user has selected a program, the system performs the following actions:
A program is not supposed to return to the caller: upon completion it typically resets the TI-99/4A. This is achieved by BLWP @>0000 in assembly and by EXIT in GPL.
A subprogram is what you call with the Basic instruction CALL. They can also be called from assembly or any other language. Theoretically there is not limit to the length of a subprogram name, but TI-Basic and the console GROM DSRLNK routine (G@>0010) set a limit of 7 characters (probably because they save the name into the scratch-pad area at >834A-8351).
The calling convention is that the subprogram name (with a leading lenght byte) should be placed in VDP memory, with word >8356-57 pointing at the length byte. Then DSRLNK can be called from GPL with CALL G@>0010.
If you're curious, here is a disassembly of XML >19 and XML >1A
Unfortunately XML >19 and XML >1A cannot be called from assembly since they return to the GPL interpreter. Therefore most assembly language package provide a custom DSRLNK routine that will search the card ROMs. The editor/assembler cartridge also provides an XMLLNK routine and a GPLLNK routine that will allow the programmer to call G@>0010. But that's because it's a GPL cartridge and it contains an XML instruction to return to the assembly caller.
There are ways around this drawback though. Here is a fairly crude one:
* This routine messes up the GPL interpreter
TXML19 MOV @>83FA,R9 Save GROM base
MOV @>83C4,R12 Save interrupts hook
MOVB @>8373,R7 Save sub stack ptr
CLR @>83D0 To search all cards
LWPI >83E0 GPL workspace
RETPT LWPI WREGS Back to our workspace
What this routine does is to change the GROM base used by the GPL interpreter to fetch the next opcode. It changes it to an address in ROM that contain byte >0A, which corresponds to the GPL instruction GT (it could also be >09, >0C or >0D. Just make sure the byte address is lower than >9800).
Then it installs an ISR hook and calls XML >19 which will scan card ROMs for the subprogram or the DSR we want to call. Whether it was found or not, XML >19 returns to the GPL interpreter. Since the "GROM base" now points at a >0A byte, the interpreter will endlessly execute a GT instructions. I chose GT because it's pretty harmless and 1-byte long.
As the interpreter enables interrupts between each instruction, our hook is prone to be branched at sooner or later. We then perform some clean up to restore the interpreter in its normal state. Finally we check whether the subprogram (or the DSR) was found.
Now, here is a more subtle way. It implements a GPLLNK routine that allow an assembly language program to call a GPL routine. It makes use of carefully selected bytes in the console GROMs to simulate an XML instruction and regain control once the GPL routine returns.
* These routines are used to call GPL and return to assembly
* This routine calls a GPL subroutine that returns with RTN or RTNC
GPLNK1 LI R1,>8373 Subroutine stack ptr
* This routine emulates a CALL G@>0010.
GPLNK2 LI R1,>8373 Subroutine stack ptr
* This routine calls an XML that returns to the GPL interpreter
XMLNK2 LI R1,>8373 Subroutine stack ptr
XMLNK1 CLR R1 No need to restore anything
* Calling examples
GPLLNK first places a dummy return address on the subroutine stack. This is the address of a >0F byte in the console GROMs. It will be interpreted as XML by the GPL interpreter, and the next byte will be used to find the address where to branch. This may vary according to the GROM version, that's why I put these values in data words. In my console GROM address >36B8 contains >0F4B which corresponds to XML >4B and will get the vector from address >3FF6 (the XML table 4 starts at >3FE0). GPLLNK replaces this word with its return point.
It also places the desired GPL address on the subroutine stack and finally branches to the GPL interpreter using the special entry point >0016 which expects R9 msb to contain an opcode. In our case, this opcode will be >00, which codes for a RTN instruction. The GPL interpreter will therefore "return" to the address we just placed on the stack, which is the GPL routine we want to call. (I used S R9,R9 instead of CLR R9 because entry point >0016 branches to a JLT that must not be taken for RTN).
If this routine returns with RTN, the GPL interpreter will just branch to the pseudo XML >4B instruction in GROM, and this will return control to our GPLLNK routine.
An alternate subroutine, GPLG10 is provided to emulate a call to G@>0010, which the the GPL equivalent of DSRLNK. It saves the GROM base on the stack together with the return address. Subprograms called in this way return with CALL G@>0012 which fetches the base and the address from the stack (all TI-Basic subprograms and DSR do so).
XMLLNK emulates an XML instruction and calls an assembly language subroutine. It is assumed that this routine will return to the caller. Unfortunately many XMLs in the console ROM don't bother to save the return address and just branch to the GPL interpreter.
GPLXML it therefore provided to call an XML that returns to the GPL intepreter, and nevertheless regain control afterwards. As it modifies the current GROM/GRAM address, you may want to save this address first, and restore it afterwards.
And now that we can call GPL routines, let's scan for subprograms the way TI-Basic does: by calling DSRLNK at >0010 in the console GROM. There is only one problem: DSRLNK expects a byte of data after the call: >08 for DSR and >0A for subroutines. Since we are not calling it with a CALL from GPL, the FETC @>836D instruction will return a dummy value. Therefore, OURLNK enters DSRLNK by skipping the FETC instruction, with the proper value already placed in >836D. Unfortunately address >0010 only contains a branch instruction to the real start point of the routine. We'll have to get the branch first, to be able to change the entry point.
* This routine makes use of GPLLNK to call G@>0010 which scans
MOVB @>837C,R1 Upon return: See if subprogram was found
Assembly subprogram called with XML >19 or an equivalent routine are entered with the GPL worskpace >83E0. The following values can be expected:
R1: Number of times the subprogram was called (normally 1).
R9: Address of the subprogram
R11: Return address (to keep scanning)
R12: CRU base address of the card.
>83D0-D1: CRU base address of the card
>83D2-D3: link to next subprogram in the header
The most usefull are R1 and R12. They allow the possiblility of installing several identical cards in the PE-Box, each with a different CRU address (generally DIP-switch selected). The subprogram can use R12 to know the CRU of the card it is in and R1 to know how many cards were found before. The RS232/PIO card works this way.
GPL subprograms will find similar informations in the scratch-pad:
>836C: Number of times the subprogram was called
>83D0-D1: GROM base (normally >9800)
>83D2-D3: link to next subprogram in the header
Assembly language subprogram typically return with::
This prevents the scanning routine (XML >19 or a clone of it) from scanning further cards for the same subprogram. Unless of course this is precisely what you want to do. In this case, just return with:
GPL subprogram often return with:
since they can expect to have been called by the GPL DSRLNK routine located at >0010 in the console GROM.
Contrarily to DSRs, there is no conventions as how to pass parameters to a subprogram. It depends on the language from which it is called. As a consequence, you'll probably have to write several versions of your subprogram, one for each language it can be called from (or at least have different entry points). For instance, the disk controller card contains a subprogram called "FILES" to be called from TI-Basic and an almost equivalent one called >16 to be called from assembly (the only difference is that you can have upto 16 files in assembly, but only 9 in Basic).
If the caller is assembly language, the parameters are generally passed on the scratch-pad (so as not to depend from the presence of a memory expansion card). Convenient addresses for that purpose are >834A-8353. If that's not enough space, you could pass a pointer to a VDP memory address in these bytes. Byte >8350 is often used to return an error code: >00 means no error, other values indicate an error.
Which means you should provide the user with a good doc, so that he/she knows how to call your subprogram. Don't forget to explain the error codes, if any.
The disk controller card subroutine >12 (file protect/unprotect)
expects the following parameters.
>834C: drive number, from >01 to >03
>834D: protection code: >00 = unprotect, >FF = protect
>834E-834F: an address in VDP memory where to find a 10-characters filename.
It returns an error code in >8350 if something went wrong.
Things get really messy when parameters must be passed from Basic or Extended basic. That's because the CALL statement does not perform any parameter passing. Therefore, your call must parse the Basic statement to extract the parameter specifications. If these are variables you must then get their values from the Basic symbol table. That's not easy to do, to say the least.
Let me first mention a simple trick that makes the programer's life
easier, but puts some burden on the user. Just tell the user to pass
after the subprogram name, separating them with a dot. For instance:
That's somewhat annoying for the user because CALLs cannot be placed in a variable: CALL A$ is not allowed. Therefore the user must enter a different line for each desired value of a parameter. In the above example, it A can have 16 different values, it means a ON A GOTO to 16 different CALL MYSUB.A=, each with a different value of A...
The user will certainly prefer this kind of syntax:
But now the burden is on you!
First you must parse the basic statement, looking for parameters. Basic codewords are not stored as such in memory: to save place they are converted to 1-byte tokens. For instance, the above statement is encoded as:
VDP address Token/bytes Meaning
>37B4 >17 Line size
>37B5 >9D CALL
>37B6 >C8 Unquoted string
>37B7 >05 String length
>37B8 MYSUB The name is of course not encoded
>37BD >B7 (
>37BE A Variable names are not encoded
>37BF >B3 ,
>37C0 >C7 Quoted string "..."
>37C1 >05 String length
>37C2 TEST2 Content of the string
>37C7 >B3 ,
>37C8 U$ Another variable
>37CA >B3 ,
>37CB >C8 Unquoted string
>37CC 512 Numeric constants are passed as strings
>37CF >B6 )
>37D0 >00 End-of-line mark
You'll find a pointer to the current token in word >832C, and the last processed token is copied in byte >8342. In this example, it will be the >C8 token following the CALL and >832C will contain >37B7.
TI-Basic programs can be stored in VDP memory or in GRAM. To know where it is, test byte >8389: if it's >00 the program is in VDP memory, otherwise it's in GRAM. Extended basic programs can't be in GRAM but they can be stored in the high memory expansion, if it's present. In this case, a non-zero value in byte >8389 means that the program is in the memory expansion.
A convenient way to avoid all this hassle is to use XML >1B: it will get the next token (or character) from the adequate memory, put it in >8342 and increment >832C accordingly.
But first, you must skip the name of your subprogram (easy: you know it's 5 chars long, so just add 5 to >832C). Then make sure there is a parenthesis, fetch each parameter, check for commas and for the closed parenthesis. If you're using GPL, a PARS >B6 instruction comes handy for this job. Finally you must return with >00 in >8342 otherwise Basic will complain.
But wait, when I said "fetch parameters" I kind of overlooked several problems.
If the parameter is a string constant ("TEST2" in the example above), no problems: just use it as it is. Almost, that is: the user may have the bad idea to concatenate two string constants with the & operator (e.g. "TEST"&"2"). You must therefore test for this situation: the token for & is >B8.
If its a numeric constant (512 in the example), you'll have to convert it to a number. That can be easy if your subprogram accept only a limited range of values (e.g. 0 to 9: just fetch the first character and substract >30 from it). If that's not the case, you have to use a string-to-number conversion routine. Fortunately, there is one the the console ROM: XML >11 and that one can be called from assembly. Place the address of the string in >8356 and call XML >11. You'll find the result in floating point format in >834A-8352. If an error occurs, it will be announced in byte >8354. You'll also find the sign in >8375 (>00 positive, >FF negative) and the exponent in >8376, if you have a need for these.
And what about variables? Oh boy, that's where it becomes really complicated. You must first call XML >13 that will search the Basic symbol table for the variable whose name is pointed at by >832C. If it finds the variable, it places its address in the Basic value stack in word >834A-834B. However, if it does not find it, it returns to the GPL interpreter to announce an error in Basic!
Now you must get the value of that variable. This is achieved by XML >14: it dumps the value stack entry for the variable pointed at by >834A-834B into bytes >834A-8351. When it returns, you'll find a pointer to the value (in VDP memory) in word >834E. But first you must test byte >834C: it will be >00 for a numeric variable and >65 for a string variable. You may also test >834D: it contains the number of dimensions for array variables (>00 of its not an array). For non-array string variables, you'll find the length of the string in word >8350. I'm not sure it's the case for string arrays. Anyone?
Oh, and don't forget that here also the user may have used the concatenation operator & to combine several string variables: you'll have to get each and everyone of them using the above method. Of course, you could just forbid the use of this operator and return with a syntax error...
Finally, you may want to convert floating point numbers to integers. This can be done with XML >12. It will place the result in word >834A-834B and the error code in >8354 (>03 if the number is not in the range -32768 to +32767). A quick trick though: if a floating point number is zero, the first word (in >834A) will be >0000, so no conversion is necessary.
Ok, will you remember all this? Well, I warned you: parameter passing is a mess with subprograms.
And just to depress you a little more: I'm not sure XML >13 and XML >14 apply to Extended Basic (in fact I think they don't...).
Ok, here is an example of parameter passing to a GPL subprogram called from Basic
* This GPL snippet illustrate parameter passing from Basic to a subprogram
MYSUB CLR @>834A Skip our subprogram name in the basic program
XML >1B Get next token
CEQ >B3,@>8342 Is next token a , ?
NOCOMA CEQ >B6,@>8342 Is there a ) ?
ERRSN CALL G@>001C Call error routine (returns to Basic)
ERRIS CALL G@>001C Call error routine (returns to Basic)
Here again it's easy for subprograms called from assembly: just place the value (or a pointer to it) in the scratch-pad.
Things are more complicated in Basic, mainly because of string variables. If you want to return a string, you cannot be sure that it will fit in the memory currently allocated for the string variable. Most probably it won't, as the user will call your subprogram with an empty string variable. You must therefore use XML >15 to assign a value to a variable, whether string or number.
To call XML >15 you first make a copy of the variable entry in the value stack (as returned by XML >14) at the top of the value stack. This is achieved by calling XML >17.
Then for numeric variables, just place the new floating point value on >834A-8352 and call XML >15: it will transfer the value in the proper memory location. Uh-oh, we have a problem here: there is no XML to convert an integer to a floating point number so you'll have to do it yourself!
For string variables, you must first get enough room for your string in the Basic string space. There is a GPL routine that does this: place the required number of bytes in >830C-830D and CALL G@>0038, the address of the allocated space will be returned in >831C. You can easily call it from GPL, but if your subprogram is written in assembly language, this may be a problem. You could either use the GPLLNK routine outlined above, or simulate G@>0038 with an assembly routine.
* This routine is an assembly version of the GPL routine >0038
STRSPA DATA WREGS,STRSP1
STRAP1 ORI R15,>2000 Set Eq bit
Now you can place your string in the VDP memory at the designated address.
Then get the Basic variable entry by calling XML >13 and XML >14 successively and push it again on the Basic value stack by calling XML >17.
Then modify it so that it points at your string in the string space. Finally call XML >15 and that's it (wow).
In summary, this is how to return variables from a GPL subprogram. All these XMLs can be called from assembly, so you should not have too many problems to come up with an equivalent version in assembly language. I'll leave it to you as an exercise.
* This GPL routine demonstate how to return a value in a Basic string variable
SENDST DST >000E,@>830C Put string size here
MOVE @>830C,G@TESTRI,V*>831C Now put a string into that space
XML >1B Get first char of variable name (required by XML >13)
DST >001C,@>834A New entry will be a string expression (flag >001C)
TESTRI TEXT 'THIS IS A TEST'
* This GPL routine demonstrates how to return a value in a Basic numeric variable
SENDNM XML >1B Get first char of variable name
DST >4101,@>834A Now put a floating point number on >834A-8451
XML >15 Assign it to the variable
ERRSN CALL G@>001C Call error routine (returns to Basic)
ERRIS CALL G@>001C Call error routine (returns to Basic)
* This is a sample GPL subprogram using the two routines above.
DSR stands for Device Service Routine. They are special subprograms meant to access custom functions provided by the card. You could call them "drivers" to follow the PC lingo. The main difference between DSRs and subprograms is that DSRs use the concept of files as a standard way of passing parameters. Therefore, in Basic DSRs are called by file operations (OPEN, CLOSE, PRINT, INPUT, DELETE, OLD, and SAVE). They can also be called from assembly, using a PAB to pass parameters to/from the DSR. DSR names can have from 1 to 7 characters.
DSRs are called the same way as subprograms. Most of the time, the same scanning routine performs handles both types of calls: all it needs is a parameter to get the first link in a header. This parameter is >0008 for DSRs and >000A for subprograms (see structure of a header, above).
The situation upon entry in a DSR is therefore just the same as for subprograms.
Most of the time, a DSR will be able to execute a variety of operations (open a file, close it, etc). The first byte in the PAB is used to pass an opcode, that specifies which operation is to be executed. DSRs are thus more complex that most regular subprograms, but paradoxically easier to use with Basic.
Just like subprograms, assembly language DSRs typically return with::
This prevents the scanning routine from scanning further cards for the same subprogram. Unless of course this is precisely what you want to do. In this case, just return with:
GROM DSRs, written in GPL, generally return with:
since they can expect to have been called by CALL G@>0010 or the assembly language equivalent of it.
The convention to pass parameters to a DSR is to use a peripheral access block (PAB). This is a bunch of data, generally stored in the VDP memory, that is used to pass parameters to and from the called DSR or subprogram. A PAB has the following structure:
|1||Err flag + File type|
|2||VDP buffer address|
|5||Number of chars|
|6||Rec # / file size|
|8||Bias / status|
This byte is used to define the type of operation to be performed by the DSR. The standard opcodes for file operations are the following:
Some DSR accept other custom opcodes. For instance, the RS232 DSR accepts:
>80 Interrupts. A variant of Open that allows for interrupts upon reception of a char on a serial port.
The Horizon Ramdisk supports several additional opcodes:
>0A Assembly. Loads and runs a memory-image assembly file.
>0B Basic. Loads and runs an extended basic file.
>0C Cartridge. Loads a memory image GPL file.
>B0 Rambo. Selects a memory bank to be mapped at >6000-7FFF.
>80 to >8C: same as >00 to >0C, but uses a buffer in CPU memory rather than in VDP memory.
The last 5 bits are used by the caller to define the file type. Typically used by Open only.
|3||>00: Fixed >10: Variable|
|4||>00: Display >08: Internal|
|5||>00: Update >04: Input
>02: Output >06: Append
|7||>00: Seq >01: Relative|
The first 3 bits are used by the DSR to return an error code:
These two bytes point to a data buffer, generally in VDP memory.
Read: write data from the file/peripheral into this buffer, record
Write: take data from this buffer to write it on file, record by record.
Load: write data from the whole file/peripheral into this buffer in a single operation.
Save: take data from this buffer to write it on file in a single operation.
For "fixed" files, this is the absolute record length, in bytes. For "variable" files, it is the maximum allowable record length. This byte is not used for "program" files.
Open: When this byte is >00, replace it with the default record
Else check if length is ok with that peripheral.
Read: Maximum number of bytes to transfer.
Read: how many bytes have been transfered from the peripheral to the
Write: how many bytes must be transfered to the peripheral.
Rewind: point on that record, for next operation (must be Relative,
unless record # is 0)
Scratch: delete that record.
Load: maximum number of bytes that can be loaded into the memory buffer.
Save: number of bytes to write to the file/peripheral.
Basic uses this byte to specify a screen bias, i.e. a value to be added to each character in case the DSR wants to display an error message on screen (the Basic bias is >60). This is typically not used as DSRs do not write directly on screen (the CS1 and CS2 DSRs are exceptions to this rule)..
Status: Return the file status in that byte:
|Bit||Meaning when 0||Meaning when 1|
|0||>00: File exists||>80:File not found|
|1||>00: Unprotected||>40: File/device protected|
|3||>00: Display||>10: Internal|
|4||>00: Non-program||>08: Program|
|5||>00: Fixed||>04: Variable|
|6||>00: Enough space||>02: Memory full|
|7||>00: Whithin file||>01: End-of-file|
N.B. Some DSR return the status after each and every opcode (The Horizon Ramdisk does so), but that's not required by the TI specs.
The size in bytes of the DSR name, including file name and modifiers.
The DSR name should start from that byte. Valid DSR names must be 1-7 characters in lenght and should not contain decimal points. Exemples: DSK1, RS232/1.
Additional parameters can be appended to the DSR name, using decimal points as separators. E.g. a filename DSK1.MYFILE or transmission parameters RS232.BA=4800.DA=8.CR.CH
What comes after the first decimal point depends on the DSR and is not part of the PAB convention.
Interrupt service routines (ISRs) only exist in peripheral card ROMs, since the GPL interpreter allows interrupts: a GPL ISR would thus almost certainly be interrupted by a VDP interrupt!. When the main ISR in the console ROM determines that an interrupt did not come from the VDP, nor from the TMS9901 timer, it assumes it came from a peripheral card. It then scans all CRU addresses from >1000 to >1F00 and branches to each and every ISR it finds in any card ROM.
It is the responsability of the card's ISR to determine whether the interrupt came from that card or not.
An ISR always returns with B *R11, so that the main ISR can scan other cards. If however you would like to make sure no other card will be scanned, here is how to return:
C *R11+,*R11+ Increments R11 by 4: jumps out of the scanning loop
We can easily jump out of the ISR scanning loop by incrementing R11 by 4. However, this skips an SBZ 0 instruction and results in leaving our card ROM "on", which most probably will crash the system. We must therefore execute this SBZ 0 ourselves. But this instruction cannot be placed in the card ROM since we won't be able to execute a program in there once the ROM is turned off. Therefore, the SBZ 0 and a B *R11 are copied into the scratch-pad into R0 and R1. We then continue execution of the program in our worspace, and execute these two instructions.
There is one more feature for GROMs: if GROM >6000 in base >9800 has a negative version numbe (i.e. bit 0 is >80), then the power-up routine will branch at >6010 just after is finished drawing the TI title screen. This was means to implement a foreign language translator, that would replace titles in english with titles in whatever language the cartridge provided. Some game cartridges use this trick to take over control of the TI-99/4A, just like some do with the power-up routines.
After the user pressed a key and the main program menu is displayed, another translator kicks in. Its address is >6013 in base >9800. This leaves you only 3 bytes to install the first translator, but that's all you need for a B G@>xxxx statement. This second translator is in charge of translating the words "PRESS" and "FOR" in the menu (it could possibly translate program name, but that would be quite a task...).
Back to the TI-99/4A Tech Pages