.OS
    .CODE
    .ADDR x8000
    JMP TRAP_GETC
    JMP TRAP_PUTC
    JMP TRAP_GETS
    JMP TRAP_PUTS
    JMP TRAP_GETC_TIMER
    JMP TRAP_DRAW_RECT

    .ADDR x8200


.CODE
OS_START
    CONST R7, #0
    RTI

.CODE
TRAP_GETC
    CONST R0, #0
    HICONST R0, #254
    LDR R0, R0, #0
    BRzp TRAP_GETC
    CONST R0, #2
    HICONST R0, #254
    LDR R0, R0, #0
    RTI

.CODE
TRAP_PUTC
    CONST R1, #4
    HICONST R1, #254
    LDR R1, R1, #0
    BRzp TRAP_PUTC
    CONST R1, #6
    HICONST R1, #254
    STR R0, R1, #0
    RTI

.CODE
TRAP_GETS
    CONST R1, #0
    CONST R4, #0
    HICONST R4, #32
    CMP R0, R4
    BRn DONE_GETS_NO_WRITE
    CONST R4, #255
    HICONST R4, #127
    CMP R0, R4
    BRp DONE_GETS_NO_WRITE
    CONST R4, #0
    CONST R4, #0

.CODE
READ
    CONST R2, #0
    HICONST R2, #254
    LDR R2, R2, #0
    BRzp READ
    CONST R3, #2
    HICONST R3, #254
    LDR R5, R3, #0
    CONST R6, #0
    HICONST R6, #13
    CMP R5, R6
    BRz DONE_GETS
    CONST R6, #10
    CMP R5, R6
    BRz DONE_GETS
    CONST R6, #0
    CMP R5, R6
    BRz DONE_GETS
    ADD R4, R0, R1
    STR R5, R4, #0
    ADD R1, R1, #1
    BRnzp READ

.CODE
DONE_GETS
    CONST R6, #0
    HICONST R6, #0
    ADD R4, R0, R1
    ADD R4, R4, #1
    STR R6, R4, #0

.CODE
DONE_GETS_NO_WRITE
    RTI

.CODE
TRAP_PUTS
    CONST R4, #0
    HICONST R4, #32
    CMP R0, R4
    BRn DONE_PUTS
    CONST R4, #255
    HICONST R4, #127
    CMP R0, R4
    BRp DONE_PUTS
    CONST R4, #0

.CODE
LOAD
    ADD R2, R4, R0
    LDR R1, R2, #0
    CMPI R1, #0
    BRz DONE_PUTS
    CONST R2, #4
    HICONST R2, #254
    LDR R2, R2, #0
    BRzp LOAD
    CONST R3, #6
    HICONST R3, #254
    STR R1, R3, #0
    ADD R4, R4, #1
    BRnzp LOAD

.CODE
DONE_PUTS
    RTI

.CODE
TRAP_GETC_TIMER
    CONST R1, #10
    HICONST R1, #254
    CONST R3, #8
    HICONST R3, #254
    CONST R2, #208
    HICONST R2, #7
    STR R2, R1, #0

.CODE
AWAIT_LOOP
    LDR R4, R3, #0
    BRn DONE_WAITING
    CONST R0, #0
    HICONST R0, #254
    LDR R0, R0, #0
    BRn READY_TO_READ
    BRz AWAIT_LOOP

.CODE
READY_TO_READ
    CONST R0, #2
    HICONST R0, #254
    LDR R0, R0, #0

.CODE
DONE_WAITING
    RTI

.CODE
TRAP_DRAW_RECT
    STR R7, R6, #-2
    STR R5, R6, #-3
    ADD R6, R6, #-3
    ADD R5, R6, #0
    ADD R6, R6, #-5
    STR R0, R5, #-1
    STR R1, R5, #-2
    STR R2, R5, #-3
    STR R3, R5, #-4
    STR R4, R5, #-5
    CMPI R2, #0
    BRnz DONE_DRAWING
    CMPI R3, #0
    BRnz DONE_DRAWING
    CMPI R1, #0
    BRnz DONE_DRAWING
    CONST R7, #124
    CMP R3, R7
    BRp DONE_DRAWING
    CONST R7, #128
    CMP R2, R7
    BRp DONE_DRAWING
    CMPI R0, #0
    BRn DONE_DRAWING
    CMPI R1, #0
    BRn DONE_DRAWING
    ADD R3, R3, R1
    ADD R2, R2, R0

.CODE
VALIDATE_SIZE_AND_POSITION_Y
    CONST R7, #124
    CMP R3, R7
    BRp FIX_Y

.CODE
VALIDATE_SIZE_AND_POSITION_X
    CONST R7, #128
    CMP R2, R7
    BRp FIX_X
    BRnz SET_UP_NEW_ROW

.CODE
FIX_Y
    SUB R7, R3, R7
    SUB R0, R0, R7
    SUB R3, R3, R7
    BRnzp VALIDATE_SIZE_AND_POSITION_X

.CODE
FIX_X
    SUB R7, R2, R7
    SUB R0, R0, R7
    SUB R2, R2, R7

.CODE
SET_UP_NEW_ROW
    CONST R7, #128
    MUL R7, R1, R7
    CONST R4, #0
    HICONST R4, #192
    ADD R7, R7, R0
    ADD R7, R7, R4
    ADD R4, R0, #0
    BRnzp UPDATE_ROW_IF_DONE

.CODE
DRAW_COLUMN
    STR R2, R5, #-3
    LDR R2, R5, #-5
    STR R2, R7, #0
    LDR R2, R5, #-3
    ADD R4, R4, #1
    ADD R7, R7, #1

.CODE
UPDATE_ROW_IF_DONE
    CMP R4, R2
    BRn DRAW_COLUMN
    ADD R1, R1, #1
    CMP R1, R3
    BRn SET_UP_NEW_ROW

.CODE
DONE_DRAWING
    ADD R6, R5, #0
    ADD R6, R6, #3
    LDR R5, R6, #-3
    LDR R7, R6, #-2
    RTI

    .DATA
    .ADDR xA000
OS_GLOBALS_MEM
NOP
NOP
NOP
NOP
NOP
NOP
NOP
NOP
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NOP

    .ADDR xC000

OS_VIDEO_MEM
NOP
NOP
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NOP
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NOP
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NOP
NOP