TI-99 Platform Specific Series

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Introduction to the Platform Specific Series...
In this series we'll be looking at examples that use the TI-99 hardware.

Lesson P1 - Bitmap graphics on the TI-99 The Ti-99 screen is basically the same as the MSX1 (The TMS9918) - it has two areas of VRAM which define tiles - one set for the bitmap data - and one set for the colors

Ti_Bitmap.asm Ti_Bitmap_Advanced.asm

To select a column of the Keymap we use CRU Address 0024h, we can then read in from the address 0006h

We can read a whole column into R4 with ST R4,8

We can test a single bit with TB n - where n is the bit (offset from 0006h) to test 

CRU Address	Col 0	Col 1	Col 2	Col 3	Col 4	Col 5	Col 6	Col 7	Bit
0006h	=	.	,	M	N	/	J1-Fire	J2-Fire	0
0008h	space	L	K	J	H	;	J1-Right	J2-Right	1
000Ah	enter	O	I	U	Y	P	J1-Left	J2-Left	2
000Ch		9	8	7	6	0	J1-Down	J2-Down	3
000Eh	fctn	2	3	4	5	1	J1-Up	J2-Up	4
0010h	shift	S	D	F	G	A			5
0012h	ctrl	W	E	R	T	Q			6
0014h		X	C	V	B	Z			7

Basics of the TMS9919

The sound chip takes all its data from a single 8 bit port... at memory mapped address 8400h
The 7th bit is the 'latch bit' which tells the chip if a new command is being sent (1) or a second part of the last command (0)
Bits 6 and 5 are the channel number... 0-2 are tone chanels, 4 is the noise channel
Bit 4 is the 'Type bit'... 0 defines the sound... 1 defines the Volume

The purpose of the remaining bits (0-3)  vary depending on the first 4

The sound port is 8400h... we load this into R1 - this means we can send sound data to reg *r1 First we silence the sound chip - we need to do this by setting the volume to 15 (higher is quieter)... we define a volume setting with bit 4- the volume is defined by the bottom 4 bits... the channel number is bits 5 and 6 Chibisound uses two channels... channel 2 for tones and channel 3 for noise. Chibisound uses a 1 byte parameter in R0 - if it's zero, then we're done, as we're not playing any sound
We're going to set the volume of Channel 2 - to do this Bit 7 and Bit 4 need to be 1. What happens next depends on if we want to make a tone or a noise
If we're making a tone we'll set up the tone channel pitch... to define the tone we first write a byte with bit 7 as 1, and bit 4 as 0... the bottom 4 bits are the low bits of the tone... we're defining channel 2 Next we send the second byte - with bit 7 as 0 - bits 0-5 are the high bits of the tone.
If we're making a noise then we need to do two things. We need to set up channel 3 for noise - but channel 3 will be linked to channel 2 There fore we still need to set the tone of Channel 2 (though we'll mute it) We then set up the noise - we set the noise type to 1 which means White Noise... we set the rate to 3 which means the rate is taken from channel 2 finally we silence channel 2 and set the volume for channel 3