Raytracing

Set the camera at a point in space and the direction that it is facing. At a distance D set a viewport through which the camera see. The viewport defines the part of the scene to be render on the canvas which is a memory buffer sized for a certain resolution. 

The scene is a 3-D space which contains objects. Each object (like a sphere) is model mathematically by equation. Each object has a distance away from the camera (D1). Object behind the viewport will be raytraced and rendered on the canvas. From the camera position to each grid on the view port forms a vector. Extend the vector directionally intent hit an object in the scene (intersect). Paint the grid with the color of the object. As the one t is 3-D, the ray may intersect multiple points on the object. Only the front most point will be painted. 

SPIE set up

The SPIE macro expanded into a PICA structure and a SVC 14 call.  PICA contains a new program mask, the address of the exit routine supplied by the caller and the program check interrupts to trap. 

SPIR is not a type 1 SVC and so a SVRB is created by FLIH to save the registers of the caller. SVC 14 will check if this is a request to eliminate or to cancel an existing SPIE set up. If not, it will check if it is the first SPIE to set up by checking the TCBPIE field for 0. It GETMAINto create a SCA and a PIE. SCA is made to point to PIE which points to PICA. 

if there is an existing SPIE, put the address of the old PICA in R1 and change the PICA to point to the new PICA. 

Lastly SVC 14 gets the program mask from RBOPSW and save it into TCBPMASK which is the first 4 bits in the first byte o TCBPIE. IT THEN BRACH TO EXIT PROLOG. 

Program interrupt handler

Among the various program.interrupts, 4 of them can be disabled (arithmetic overflow or underflow). MVS will ignore these interrupts when it is disabled. PCFLIH determine if there is a SPIE set up by user to handle the specific types of interrupt and passes control to it if it does. This thereise PCFLIH passes control to OS EXIT routine will will generate a dump if SYSABEND or SYSDUMP DD card is specified. 

SVC EXIT PROLOG

 After SBC completed processing, it passed control to EXIT PROLOG via a BR 14. EXIT PROLOG may pass control to the dispatcher to f the force switch is on. Done SVC like POST or ENQ will caused new work to be ready and thus letting the dispatcher to chose the highest priority work to run next. If the force switch is ofF, EXIT PROLOG may pass control back to the caller. T first determine the type of SVC to work out how to restore the state before returning. Type 1 saved the register in TCB and the other type saves in SVRB. 

EXIT PROLOG would turn off the type 1 flag in ASCB and releases all locks used by SVC. For other SVC types, it acquired the local lock so it can manipulate the RB chain, restored the registers and disabled the io and external interrupt to make it same condition at the time of interrupt. It uses a LPSW instruction to pass control back to the caller. 

web attacks

Injection is to trick the server side to execute code by appending code fragment to the input.  If server side use the input to form the final call to the system, the code fragment is executed.  SQL injection add additional SQL code fragment.  Command injection happens when the system call uses the input.   Remote code execution uses similar principle to exploit web server handling of input.

Cross site scripting attack (XSS) on the other hands is to inject code to client side (web browser).  Web browser dutifully execute all javascript code received.  Stored XSS exploit web pages that embed data stored in database.  Attacker send data contain <script> code segment to the server.  When user retrieve these data (e.g. chat group), the data will be placed inline in the final web page and the code would be executed by the web browser.  

Reflected XSS uses the HTTP request to insert code into the return web page.  Some site may include part of the URL in the web page.  To mitigate, server must sanitize the URL before embed it into the web page to return.  DOM based XSS exploit the URI fragment.  URI fragment is the part of the URL after '#' character.  URI fragment instruct the web browser to automatically scroll to the frame that matches the fragment label.  URI fragment is not saved with the URL in bookmark.  URI segment is not sent to the server and so server side cannot sanitize the URL effectively.

 

 

Color Depth

 To represent a colour on computer, each base color (RGB) is represented by a number of bit. The number of bit represent the intensity of the color emitted by variying the voltage to each colour pixel in the screen. The group of bits for a colour is called a color channel. The number of bit per colour represent the colour depth. Green colour may have more bits than red and blue in some representation because human eyes are more sensitive to green. 

Color model

Subtractive colour applies to light reflect off a surface such as paper. A colour painted on paper work by filtering out (subtract) certain frequent from white light hitting the surface. For example red color on paper is when light hit the color layer and certain frequency is removed, the remaining light hit the surface, reflected off and goes through the paint layer again. The result is the remains frequency shown as red.  The base color is not red blue and yellow, but cyan, magenta and yellow (CYM)  mixing all 3 colours result in a dark color not reallly black. The colour model is represented as CYMK and K stand for black  

Additive colour on the other hand is used when light is emitted from source.and directly observed.  This is the light sent out from monitor. The base color is red, blue and green (RBG). Mixing all 3 base colour result in white.