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Work on fleshing out sections

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Kyle Cardoza 2024-03-23 22:34:34 -04:00
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Audio & Video.md
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@ -126,3 +126,39 @@ The `IEN` register, located at `0x00DF06`, has six bits which relate to the gene
- Bit 3, called `IEN_AFLOW`, enables the interrupt triggered when the PCM sample buffer is less than 1/4 full.
- Bit 6 contains the high-order bit of the 9-bit value of the `SCANLINE` register, located at `0x00DF08`. Both this bit and the `SCANLINE` register are read-only.
- Bit 7 contains the high-order bit of the 9-bit value of the `IRQLINE` register, located at `0x00DF08`. Both this bit and the `IRQLINE` register are write-only.
### ISR
The `ISR` register, located at `0x00DF07`, contains flag bits which identify the source of the active VERA interrupt:
- Bit 0, called `ISR_VSYNC`, identifies a VSYNC interrupt condition when set.
- Bit 1, called `ISR_LINE`, identifies a raster line interrupt condition when set.
- Bit 2, called `ISR_SPRCOL`, identifies a sprite collision interrupt condition when set.
- Bit 3, called `ISR_AFLOW`, identifies a PCM sample buffer low interrupt condition when set.
- Bits 4-7 store the sprite collision data.
Writing a `1` value to bits 0, 1, or 2 will clear the interrupt state of the relevant interrupt source. Bit 3 can only be cleared by filling the PCM sample buffer to at least 1/4 of its capacity.
### IRQLINE_L / SCANLINE_L
The low order 8 bits of the current scanline and the scanline which is set to trigger the scanline interrupt share the address `0x00DF08` -- `IRQLINE_L` is write-only, and `SCANLINE_L` is read-only, so there is no conflict.
### DC_VIDEO / DC_HSTART
The `DC_Video` and `DC_HSTART` registers share the address `0x00DF09`; which one is accessed at that address is determined by the `DCSEL` flag bit in the [CTRL](#ctrl) register -- a `0` value in `DCSEL` will enable the `DC_VIDEO` register, and a `1` value the `DC_HSTART` register.
`DC_VIDEO` stores three flag bits:
- Bit 4 enables tile/bitmap layer 0 when set to `1`.
- Bit 5 enables tile/bitmap layer 1 when set to `1`.
- Bit 6 enables sprites when set to `1`.
`DC_HSTART` controls the first active column of the screen, relative to the 640x480 display area. The value is the top 8 bits of a 10-bit number, the low order 2 bits of which are implied to be 0 -- that is, the value is an 8 bit number which will be multiplied by 4.
### DC_HSCALE / DC_HSTOP
The `DC_HSCALE` and `DC_HSTOP` registers share the address `0x00DF0A`; which one is accessed at that address is determined by the `DCSEL` flag bit in the [CTRL](#ctrl) register -- a `0` value in `DCSEL` will enable the `DC_HSCALE` register, and a `1` value the `DC_HSTOP` register.
`DC_HSCALE` sets a fractional scaling factor of the active part of the screen; a value of 128 will output one pixel for each input pixel, and a value of 64 will output two pixels for each input pixel.
`DC_HSTOP` controls the first active column of the screen, relative to the 640x480 display area. The value is the top 8 bits of a 10-bit number, the low order 2 bits of which are implied to be 0 -- that is, the value is an 8 bit number which will be multiplied by 4.