161 lines
4.4 KiB
Markdown
161 lines
4.4 KiB
Markdown
# I/O bus
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The I/O bus is accessed using the `in` and `out` instructions.
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Every access is 32 bits wide; there is no byte-addressing on the I/O bus and
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addresses don't have to be aligned to multiples of four.
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When a peripheral has multiple instances (e.g. 32 display overlays, 4 disks),
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the lowest bits of the address usually indicate the instance number, while
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higher bits indicate the function to be performed.
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| start | end | description
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|------------|------------|---------------------------------------
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| 0x00000000 | 0x00000000 | serial port
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| 0x80000000 | 0x8000031f | display
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| 0x80000400 | 0x80000401 | mouse
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| 0x80000500 | 0x80000500 | keyboard
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| 0x80000600 | 0x80000600 | audio
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| 0x80000700 | 0x80000706 | RTC and uptime
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| 0x80001000 | 0x80005003 | disk
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| 0x80010000 | 0x80010000 | power controller
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## 0x00000000: Serial port
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### 0x00: Data register
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Writing to this register outputs the lowest 8 bits as a byte on the debug
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serial port (stdout in case of the fox32 emulator). If necessary, the CPU is
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stalled long enough to ensure that the byte is output rather than discarded.
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## 0x80000000: Display
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The display controller manages a background framebuffer and 32 overlays,
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and composes them into the picture seen on screen.
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The background framebuffer is located at the fixed RAM address 0x2000000, or 32 MiB.
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The screen resolution is 640x480 (307200 pixels, 1228800 bytes).
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The pixel format is RGBA. The alpha channel determines transparency in overlays
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insofar that a non-zero alpha value causes a pixel to be fully opaque and a
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zero alpha value causes it to be fully transparent.
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When multiple overlays are active at a specific pixel position, the highest
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numbered overlay that provides an opaque pixel "wins": Its pixel RGB value is
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output on screen.
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### 0x00-0x1f: Overlay position
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bits | description
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--------|------------------
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31:16 | Y (vertical) position
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15:0 | X (horizontal) position
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### 0x100-0x11f: Overlay size
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bits | description
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--------|------------------
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31:16 | height (vertical size)
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15:0 | width (horizontal size)
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### 0x200-0x21f: Overlay buffer pointer
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RAM address of the framebuffer for this overlay.
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### 0x300-0x31f: Overlay enable
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- Write non-zero to enable this overlay, zero to disable
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- Read 1 if enabled, 0 of disabled
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## 0x80000400: Mouse
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The mouse position and button states can be read and written, but hardware may
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change them on incoming mouse events.
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### 0x00: Button states
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bits | description
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--------|------------------
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2 | mouse button is currently held
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1 | mouse button has been released
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0 | mouse button has been pressed
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### 0x01: Mouse position
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bits | description
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--------|------------------
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31:16 | Y (vertical) position
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15:0 | X (horizontal) position
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## 0x80000500: Keyboard
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### 0x00: Get key event (read-only)
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Read this register to get a key event from the key event queue, or 0 if none is
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currently available.
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bits | description
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--------|------------------
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8 | 0=pressed, 1=released
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7:0 | PC-compatible keyboard scancode
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## 0x80000600: Audio
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TODO
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## 0x80000700: Real-Time Clock (RTC) and Uptime
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offset | description
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--------|------------------
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0x00 | year (e.g. 2023)
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0x01 | month (1-12)
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0x02 | day (1-31)
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0x04 | minute (0-59)
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0x05 | second (0-59)
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0x06 | milliseconds since startup
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## 0x80001000: Disk
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The disk controller supports four disk ports. Each of them may be connected to
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a disk (or not) at any time.
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Disks are organized in sectors of 512 bytes each.
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### 0x80001000-0x80001003: Disk size (read-only)
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Read this register to get the disk size in bytes.
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### 0x80002000-0x80002003: Buffer pointer
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Address of RAM buffer to be used for read/write transfers.
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### 0x80003000-0x80003003: Initiate disk read (write-only)
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Write a sector number to this register to initate a disk read from the
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specified sector. The CPU is stalled until the read completes.
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### 0x80004000-0x80004003: Initiate disk write (write-only)
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Write a sector number to this register to initate a disk write to the specfied
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sector. The CPU is stalled until the write completes.
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### 0x80005000-0x80005003: Remove disk (write-only)
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Write any value to this register to remove or eject the specified disk.
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## 0x80010000: Power controller
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### 0x00: Poweroff
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Write 0 to this register to power the computer off (or terminate the emulator).
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Poweroff does not take effect immediately, so software should do something safe
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(such as spinning in a loop) after triggering the poweroff.
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