The TI-99/4A console contains 256 bytes of RAM that appear at address >8000-80FF. As the address is not fully decoded, the same memory appears at >8100-81FF, at >8200-82FF, and at >8300-83FF. The convention is to use the >8300 addresses, however well-written programs (such as the ROM DSR in the TI cards) figure out where the RAM actually lies and never specify any "hard-coded" address.
This tiny memory is called the scratch-pad RAM and is the only CPU RAM in the console. Surprisingly, it is made of static RAM, which was a very expensive solution for the time. Probably because the TI-99/4A was originally designed around a microprocessor containing 256 bytes of RAM. When the TMS9900 was substituted for it, external RAM had to be added and SRAM was chosen because it did not require a refresh circuit like DRAM (or so I was told).
The scratch-pad is implemented as two 128 bytes chips, one holding the odd bytes, the other one holding the even bytes. Together, they make up a 16-bit data bus which results in quicker memory access. Apart for the console ROMs, this is the only 16-bit memory in the TI-99/4A system.
Vss |1 o M 24| Vcc MCM 6810P
D0/D11 |2 C 23| A6
D1/D10 |3 M 22| A5
D6/D9 |4 21| A4
D5/D8 |5 6 20| A3
D3/D15 |6 8 19| A2
D4/D14 |7 1 18| A1
D2/D13 |8 0 17| A0
D7/D12 |9 P 16| R/W*
CS0 |10 15| CS5
CS1* |11 14| CS4*
CS2 |12 13| CS3
A0-A6 Address bus. Connected to the console address bus, lines A8 to A14.
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
the range >8000-83FF is accessed. The selection is performed by a
decoder, with A0, A1 and A2 as inputs, and MEMEN* as enabling input
Output Y4* is combined with address lines A3, A4 and A5 via three
OR gates. The output of the last gate controls CS1*.
CS2, CS4*, CS5. Chip select. Hardwired to the ground in the TI-99/4A.
CS0, CS3. Chip select (active high). Hardwired to +5V in the Ti-99/4A.
R/W* Read/write selection: low for write operations, high for reads. In the console, this line is controlled directly by the WE* pin of the TMS9900 CPU.
Being RAM, the scratch-pad can contain any kind of data. However, the TI operating system in the console ROMs expects some values at precise locations. So do the GPL interpreter, the TI-Basic interpreter and the Extended-Basic interpreter. Of course other programs may use the scratch-pad at their leisure... The tables below attempt at summarizing important values.
The first part of the scratch-pad is used mainly by TI-Basic and Extended Basic. If a program does not require any of these languages, these bytes are free for use.
|>8300-8316||Temporary variables storage.|
|>8318||Used by LINK + LOAD. Also: beginning of string space.|
|>831A||First free address in VDP. Also: end of string space.|
|>831C||PAB error or temporary string pointer.|
|>831E||Start of current Basic statement.|
|>8320||Current screen address.|
|>8322||Error code returned by assembly language routines.|
|>8324||VDP value stack base pointer.|
|>8326||Return address for assembly language routines.|
|>8328||NUD table for assembly language routines.|
|>832A||Ending screen display pointer.|
|>832C||Program text or token pointer.|
|>832E||Pointer to current line number, in line number table.|
|>8330||Start of line number table pointer.|
|>8332||End of line number table pointer.|
|>8334||Data pointer for READ.|
|>8336||Line number table pointer for READ.|
|>8338||Address of intrinsic Poly constants ???|
|>833A||Subprogram symbol table pointer.|
|>833C||PAB address in VDP RAM: first link in PAB list.|
|>833E||Symbol table pointer.|
|>8340||VDP RAM free space pointer.|
|>8342||Current char/token (value).|
|>8344||Contains >FF if RUN, else >00 ( * READY * ).|
|>8345||Extended Basic flag bits: 1=1 On break next,
3=1 Trace, 4=1 Edit mode, 5=1 On warning stop,
6=1 On warning next, 7=1 Auto-num, 0&2 reserved.
|>8346||Crunch buffer destruction level (the crunch buffer is used
to convert a line of text into Basic tokens).
|>8348||Last subprogram block on stack.|
|>836C||Floating point error address in GROM ??|
|>836D||Contains >08 for DSR call.|
The second part of the scratch-pad memory is more general and heavily used by the GPL interpreter and the console ROM routines. The names in the left column are used by my GPL assembler, most (but not all) of them are generally accepted.
|FAC||>834A-8352||Floating point accumulator (8 bytes)|
|PAB||>834A-8354||PAB copy area|
|FLTERR||>8354||Floating point error code|
|DSRSIZ||>8354||Size of DSR name|
|DSRNAM||>8356||Pointer to DSR name for LINK|
|ARG||>835C-8364||Floating point argument (8 bytes)|
|VAL||>836E||Pointer to real numbers value stack in VDP mem|
|FREE||>8370||Highest free VDP memory address|
|VLPTR||>8372||Pointer to top of value stack (in pad)|
|SBPTR||>8373||Pointer to top of subroutine stack (in pad)|
|MODE||>8374||Keyboard scanning mode|
|KEY||>8375||Code if the key detected (>FF = none)
Also: sign of a real number
|JOYY||>8376||Joystick vertical value (4, 0, >FC)
Also: exponent of a real number
|JOYX||>8377||Joystick horizontal value (4, 0, >FC)|
|RANDOM||>8378||Random number, found after RND|
|TIMER||>8379||VDP interrupt timer|
|AUTO||>837A||Highest sprite in auto-motion|
|VDPSTS||>837B||Copy of VDP status byte|
|GPLSTS||>837C||GPL status byte|
|CCHA||>837D||Char at current screen position|
|CROW||>837E||Current screen row|
|CCOL||>837F||Current screen column|
|SBSTA||>8380-839F||Area reserved for subroutine stack (32 bytes, first 9 for Basic)|
|VLSTA||>83A0-83BF||Area reserved for data stack (32 bytes)|
|INTWS||>83C0||Interrupt routine workspace (32 bytes) Random number seed|
|AMSQ||>83C2||ISR disabling flags: >80 All, >40 Motion, >20 Sound, >10 Quit key|
|ISR||>83C4||Interrupt Service Routine hook: routine to be executed|
|SNDTAB||>83CC||Address of the sound table|
|SNDSIZ||>83CE||Sound bytes to play (>0100)|
|VDPR1||>83D4||Copy of VDP register 1, used by SCAN|
|CLRSC||>83D6||Screen timeout counter: decremented by 2, clears when 0|
|.||>83D8||Return address saved by SCAN.|
|.||>83DA to >83DF||used for RTWP (workspace, pc and status).|
|GPLWS||>83E0||GPL interpreter workspace|
|GBASE||>83FA||GROM port currently used (normally >9800)|
|SPEED||>83FC||Speed value, added to TIMER|
|FLAGS||>83FD||>20 cassette operations, >10 cassette verify, >08
>02 multicolor mode, >01 sound table in VDP mem
|.||>83FE||VDP write address port (>8C02).|
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