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149 lines
5.1 KiB
Markdown
149 lines
5.1 KiB
Markdown
# Videotron2K
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Videotron2K emulates hypothetical video display controller where target video device has framebuffer and the controller has a 1 scanline buffer and 16 registers and programmed in specialised assembly.
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When program is running, video display controller must maintain the output signal (unless explicitly blanked, this behaviour depends on the display controller) and when the program ends, screen must be blanked.
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## Registers
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Videotron2K has 6 general and 4 special register, of which
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- r1 through r6 : general register of 32 bit signed integer
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- tmr : an RTC of signed integer, resolution of a milisecond
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- frm : a frame counter of 32 bit signed integer
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- px and py : a cursor where drawing operation is happening
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- c1 through c6 : counter variable in 32 bit signed integer
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Most commands accept pseudo-register of '0', which returns 0 when read and ignores any writing attempts.
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The system uses two's complement for negative numbers.
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## Assembly Format
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Consider the following code snippet:
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```
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DEFINE RATEF 60 ; 60 fps
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DEFINE height 448
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DEFINE width 560
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SCENE initialise
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@ mov r1 height ; this line runs only once when the SCENE is called
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fillin 254 r1 0 width ; fillin fills entire scanline.
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; Syntax: fillin pixel-value scanline x-start x-end-exclusive
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; final (px,py) will be (scanline,x-end-exclusive)
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dec r1
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exitzr r1 ; if condition is not met, the next line runs
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; if next line is not there, it goes to the first non-@ line
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END SCENE
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SCENE 0 ; indexed scene
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goto 100 100 ; moves pixel cursor to (x,y) = (100,100)
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plot 1 54 231 7 82 64 22 5 ; writes following bytes into the framebuffer
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END SCENE
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SCENE 1 ; indexed scene
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goto 100 102 ; moves pixel cursor to (x,y) = (100,102)
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plot 231 1 54 17 182 62 2 35 ; writes following bytes into the framebuffer
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END SCENE
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SCENE anim
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@ define cnt 2 ; definition of the local constant
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@ mov c1 0
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perform c1 ; accessing the indexed scene
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inc c1 ; slightly inefficient way to make comparision
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cmp c1 cnt r1 ; slightly inefficient way to make comparision
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exitzr r1
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END SCENE
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perform initialise ; this command executes whatever defined in the target scene
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; perform only accepts scene name
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perform anim
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goto 0 447
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plot 0 0 254 254
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next ; advance a frame counter (frm) and sleeps until it is time to draw next frame
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exeunt ; this explicitly ends the program
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```
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### Conditional Postfixes
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Commands can have "conditional postfix" used to execute the command conditionally.
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- zr r : if r == 0
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- nz r : if r != 0
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- gt r : if r > 0
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- ls r : if r < 0
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- ge r : if r >= 0
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- le r : if r <= 0
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## Available Commands
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### Arithmetic
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* add rA rB rC : rC = rA + rB
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* sub rA rB rC : rC = rA - rB
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* mul rA rB rC : rC = rA * rB
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* div rA rB rC : rC = rA / rB
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* and rA rB rC : rC = rA & rB
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* or rA rB rC : rC = rA | rB
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* xor rA rB rC : rC = rA ^ rB
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* shl rA rB rC : rC = rA << rB
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* shr rA rB rC : rC = rA >> rB
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* ushr rA rB rC : rC = rA >>> rB
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* inc rA rB : rC = rA + 1
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* dec rA rB : rC = rA - 1
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* not R : R = !R (ones' complement of R)
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* neg R : R = -R (twos' complement of R)
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### Conditional
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* cmp rA rB rC : compares rA and rB and stores result to rC. 1 if rA > rB, -1 if rA < rB, 0 if rA == rB.
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### Data Control
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NOTE: Any drawing command will clobber internal memory starting from address zero.
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* mov rA rB/I : assignes R with contents of rB/constant I
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* data rA/I bytes : writes bytes to the internal memory of address starting from rA/I
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* mcp from y x len : copies part of the internal memory to the framebuffer, from the internal memory address `from`,
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to scanline `y`, horizontal position `x`, with copying length of `len`.
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### Flow Control
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* perform scenename : gosub into the scenename
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* next : advance a frame counter (frm) and sleeps until it is time to draw next frame
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* exit : terminates current scene. System will error out if this command is used outside of a scene
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* exeunt : completely terminates the program
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### Drawing
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NOTE: Any drawing command will clobber internal memory starting from address zero.
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* fillin byte y x-start y-end-exclusive : fills entire scanline of `y` from the horizontal position `x-start` through
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`y-end-exclusive` MINUS ONE. final (px,py) will be (scanline,x-end-exclusive)
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* plot byte... : writes bytes into the framebuffer
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* fillscr byte : fills entire screen with a given byte
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* goto x y : writes `x` to px and `y` to py (use `mov px <something>` to write to px/py only)
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* border r g b : sets border colour
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### Timing Control
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* wait : waits for external signal before advancing to next frame.
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### Assembler-only
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* define : defines a global variable
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* defvar : defines a scene-local variable
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* scene name|number : defines a named scene
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* end scene : ends a scene definition
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#### Predefined Constants
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* RATET : framerate defined by miliseconds between each frame. Mutually exclusive with RATEF.
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* RATEF : framerate defined by how many frames must be shown in one second. Mutually exclusive with RATET. |