The TI-99/4A console contains two 4K ROMs that correspond to addresses >0000 to >1FFF. One of them holds the even-address bytes, the other the odd-address bytes. Together they allow for full 16-bit access from the TMS9900 CPU.
+----+--+----+
A7 |1 o T 24| Vcc TMS 4732 / 2352
A6 |2 M 23| A8
A5 |3 S 22| A9
A4 |4 4 21| CS2*
A3 |5 7 20| CS1*
A2 |6 3 19| A10
A1 |7 2 18| A11
A0 |8 / 17| D7
D0 |9 2 16| D6
D1 |10 3 15| D5
D2 |11 5 14| D4
Vss |12 2 13| D3
+------------+
Power supply
Vcc +5V
Vss Ground
CPU interface
A0-A11 Address bus. Connected to the address bus, lines A3
to
A14 in the console.
D0-D7 Data bus. In the TI-99/4A, one chip is connected to lines
D0 to D7, the second chip to lines D8-D15.
CS1* Chip select. In the TI-99/4A, active (low) when memory
in
the range >0000-1FFF is accessed. The selection is performed by a
74LS138
decoder, with A0, A1 and A2 as inputs, and MEMEN* as enabling input
G2A*.
Output Y0* is connected to CS1* of both ROM chips via a 1K resistor.
CS2* Chip select. Ditto. Hardwired to ground in the TI-99/4A..
The TMS9900 microprocessor uses the beginning of its address space for special purposes. The console ROMs contain the adequate values for (some of) these features.
Address >0000: vector for power-up (WS = >83E0, PC = >0024)
Addresses >0000 to >003F contain vectors for interrupts levels
0 to 15. In the TI-99/4A, all interrupts are hard-wired as level 1, so
only this vector is relevant. Nevertheless, a vector for level 2 is
also
present (and level 0 is the same as power-up).
Address >0004: Interrupt vector 1 (WS = >83C0, PC = >0900)
Address >0008: Interrupt vector 2 (WS = >83C0, PC = >0A92)
Addresses >0040 to >007F contain vectors for the 16 XOP
opcodes.
Only the first two are implemented for sure, although some console
carry
a third one. The remaining addresses contain assembly language, which
may
or may not result in meaningfull vectors (see discussion of the XOP
instruction in my TMS9900 page).
Address >0040: XOP0 (WS = >280A, PC = >0C1C) Calls a
never-released
extended GPL card (i.e. crashes)
Address >0044: XOP1 (WS = >FFD8, PC = >FFF8) User defined
operation.
Address >0048: XOP2 (WS = >83A0, PC = >8300). User-defined
operation.
Not on all consoles!.
After these special addresses, the console ROMs mainly contain:
The GPL interpreter,
The keyboard scanning routine,
The interrupt service routine,
Routines that scan the GROM and card ROM headers for subprograms, DSR,
etc.
Some mathematical subroutines (many
others
are in GROM),
Low-level routines for cassette
operations (DSRs are in GROM).
Here is a more detailed description of the ROM contents. For a commented listing, see Heiner Martin's book: TI-99/4A Intern (pdf 560k).
| Address | Contents |
|---|---|
| >0000 | Power up vector (>83E0, >0024). |
| >0004 | Level 1 interrupt vector (>83C0, >0900) |
| >0008 | Level 2 interrupt vector (>83C0, >0A92. Not used) |
| >000C | CPU clock speed (>28 or >30) and header mark (>AA) . |
| >000E | Address of the keyboard scanning routine (>02B2). |
| >0016 | Alternate entry point into the GPL interpreter (with an opcode in R9). |
| >001C | Alternate entry point into the GPL interpreter (with no interrupts). |
| >0020 | Address of the routine that tests the <Clear> key (>04B2). |
| >0024 | Entry point into the GPL interpreter at power-up time. |
| >0036 | Return from the (unreleased) extended-GPL board. |
| >0040 | Vectors for XOP0 (>280A, >0C1C). |
| >0044 | Vectors for XOP1 (>FFD8, >FFF8). |
| >0048 | Vectors for XOP2 (>83A0, >8300). Absent on some consoles. |
| >004E | Interpretation of GPL opcode SWGR. |
| >0060 | Alternate entry point into theGPL interpreter (with an address in R6). |
| >0070 | Main entry point into the GPL interpreter. |
| >0086 | Dispatch of the standard GPL opcodes (>80 to >FF). |
| >00CC | Interpretation of GPL opcodes CGE, CH, CHE, CGT, CLOG, CZ,
CEQ, CARRY,
OVF, H, GT, B, BS, BR, ABS, NEG, CLR, INV, FETCH, CASE, PUSH, DECT, INCT, INC, SUB, DEC, ADD, AND, OR, XOR, ST, EX, SRA, SLL, SRL, SRC, MPY, and DIV (see table below). |
| >0270 | Dispatch of the special GPL opcodes (>00 to >1F). |
| >027A | Interpretation of GPL opcodes RAND, BACK, SCAN. |
| >02B2 | Keyboard scanning routine. |
| >04B2 | Test <Clear> key (Fctn-4). |
| >04DE | Subinterpreter for GPL opcodes FMT, VCHA, HCHA, VTEX, HTEX,
IROW, ICOL,
RPTB, FEND, LOOP, ROW, COL, SCRO, HTEX (see table below). |
| >05A2 | Interpretation of GPL opcodes ALL, I/O, XML, MOVE, COINC,
RTGR, RTN,
RTNC, CALL, PUSH, and various subroutines used by the interpreter (see table below). |
| >0900 | Interrupt service routine. >0918: Peripheral cards interrupts. >094A: VDP interrupts. |
| >0AC0 | SROM (XML >19). Search card ROMs for subprograms, DSRs, or power-up routines (= DSRLNK). |
| >0B24 | SGROM (XML >1A). Search GROMs for subprograms, DSRs, or power-up routines. |
| >0C0C | Common entry point for extended-GPL opcodes. |
| >0C1C | Entry points to the never-released extended-GPL board (XOP 0). |
| >0C36 | Jump tables for the GPL interpreter. |
| >0CFA | XML master table (i.e. address of other tables). |
| >0D1A | FLTAB: jump table for XML >0x. |
| >0D3A | FCOMP (XML >0A). Floating point comparison. |
| >0D46 | SCOMP (XML >0F). Comparison in stack. |
| >0D74 | SSUB (XML >0C). Substraction in stack. |
| >0D7C | FSUB (XML >07). Floating point substraction. |
| >0D80 | FADD (XML >06). Floating point addition. |
| >0D84 | SADD (XML >0B). Addition in stack. |
| >0E88 | FMUL (XML >08). Floating point multiplication. |
| >0E8C | SMUL (XML >0D). Multiplication in stack. |
| >0F54 | ROUND1 (XML >01). Rounds real number to 14 digits. |
| >0F4A | STST (XML >03). Store status after comparing a real number to zero. |
| >0FB2 | ROUND (XML >02). Round a real number with specified number of bytes in >8354. |
| >0FC2 | OVEXP (XML >04). React to overflow/underflow (according to sign of >8376). |
| >0FCC | OV (XML >05). React to overflow. |
| >0FF4 | FDIV (XML >09). Floating point division. |
| >0FF8 | SDIV (XML >0E). Division in stack. |
| >11A2 | CSNGR(XML >11) CSN from a string in GROM or VDP. |
| >11AE | CSN (XML >10). Convert a string to a real number (and round it if needed). |
| >12A0 | Jump table for XML >1x. |
| >12B8 | CFI (XML >12). Convert a floating point number to an integer. |
| >1346 | Cassette write (GPL opcode I/O 4). |
| >1404 | Interrupt service routine used during cassette operations (meant for TMS9901 timer interrupts only). |
| >1426 | Cassette verify (GPL opcode I/O 6). |
| >142E | Cassette read (GPL opcode I/O 5). |
| >15D6 | SCHSYM (XML >16). Search a Basic variable by name . |
| >163C | Entry points for XML >17 (>163C), XML >15 (>1642), XML >13 (>1648) and XML >14 (>164E). |
| >1670 | SMB (XML >14). Get the value of a Basic variable. |
| >176A | SYM (XML >13). Search the symbol table for a variable named in a Basic statement. |
| >1788 | ASSIGNV (XML >15). Assigns value to a Basic variable. |
| >1868 | PGMCH (XML >1B). Get next Basic token. |
| >18C8 | Interpretation of GPL opcode PARSE. |
| >1920 | Interpretation of GPL opcode CONT. |
| >1968 | Interpretation of GPL opcode EXEC. |
| >19E6 | EXEC entry points for DEF, DIM, DATA, REM, OPTION. |
| >19F0 | Interpretation of GPL opcode RTNB. |
| >1A2C | EXEC entry points for "error 0", END, STOP, unquated string
(from PARSE), GO, ON, GOSUB, GOTO, RETURN, IF, LET, NEXT. |
| >1C9C | Jump table for EXEC. |
| >1CE2 | Jump table for PARSE. |
| >1DE2 | Jump table for CONT. |
| >1D3E | CONT entry points for: ( ) = + - * / ^ |
| >1E4A | Various subroutines for use by the Basic interpreter. |
| >1EAA | VPUSHG (XML >17). Pushes data (symbol description) on
Basic value
stack. Alternate entry at >1E9C, returns with CONT. |
| >1F2E | VPOP (XML >18). Pops data (symbol description) from the Basic value stack |
| >1F7E | Various subroutines used by the Basic interpreter. |
| >1FFC | ROM checksum (>2A61, >A38A). |
Detailed table of GPL interpreter routines
| Address | Opcode | Address | Opcode | Address | Opcode |
|---|---|---|---|---|---|
| >0024 | EXIT | >019A | XOR | >05C8 | I/O |
| >004E | SWGR | >019E | ST | >05D6 | I/O sound |
| >00CC | CGTE | >01A2 | EX | >05E8 | I/O cru in |
| >00D6 | CH | >01B0 | SRA | >05EA | I/O cru out |
| >00DA | CHE | >01B4 | SLL | >0608 | XML |
| >00DE | CGT | >01B8 | SRL | >061E | MOVE |
| >00E2 | CLOG | >01C2 | SRC | >06D2 | COIN |
| >00EA | CZ | >01CE | MPY | >07AA | Arguments decoding |
| >00EC | CEQ | >01EA | DIV | >082C | RTGR |
| >00F4 | CAR, OVF, HIGH, GT | >017A | RND | >0838 | RTN |
| >0104 | B | >029E | BACK | >083E | RTNC |
| >010E | BS | >02AE | SCAN | >0842 | Pop address from substack |
| >011A | BR | >04DE | FMT Subinterpreter | >085A | CALL |
| >0136 | ABS | >0502 | Fmt VCHA | >0864 | Push address on stack |
| >013A | NEG | >0504 | Fmt HCHA | >0880 | Set screen ptr to write |
| >013E | CLR | >0508 | Fmt VTEX | >0884 | Set screen ptr to read |
| >0140 | INV | >050A | Fmt HTEX | >08AA | Set address in multicolor mode |
| >0144 | FETC | >0532 | Fmt IROW | >0C0C | Extended-GPL opcodes |
| >0162 | CASE | >0534 | Fmt ICOL | >0C1A | Extended-GPL opcode >1F |
| >016E | PUSH | >053A | Fmt RPTB | >1346 | I/O cassette write |
| >0182 | DECT | >0550 | Fmt LOOP, FEND | >1426 | I/O cassette verify |
| >0184 | INCT | >0584 | Fmt ROW, COL | >142E | I/O cassette read |
| >0186 | INC, SUB | >058E | Fmt SCRO | >18C8 | PARS |
| >0188 | DEC, ADD | >0596 | Fmt HSTR | >1920 | CONT |
| >0190 | AND | >05A2 | ALL | >1968 | EXEC |
| >0196 | OR | >19F0 | RTB |