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add system-software-beck
Prithu Goswami prithugoswami524@gmail.com
Sat, 15 Feb 2020 09:36:36 +0530
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A system-software-beck/01-background.md

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+--- +geometry: +- lmargin=0.9in +- rmargin=0.3in +- tmargin=0.3in +- bmargin=0.5in +- twoside +papersize: A4 +... + +## SIC Machine Architechture + +### Memory + +- 32,768 ($2^{15}$) bytes in memory +- All addresses are byte addresses and words are addresssed by the location + of their lowest numbered byte. + +### Registers + +- Each register is 24 bits in length + +![Registers of basic SIC](img/sic-basic-registers.png){width=70%} + +### Data Formats + +- Integers are stored as 24-bit binary numbers +- There is no floating point in basic SIC + +### Instruction Formats + +- The flag bit $x$ is used to indicate indexed-addressing mode. + +![Instruction format for basic SIC](img/sic-basic-instfmt.png){width=50%} + + +### Addressing Modes + +- two addressing modes available indicated by the $x$ bit +- (X) means the contents of the register X + +![Addressing modes for basic SIC](img/sic-basic-addressing-modes.png){width=50%} + + +### Input and Output + +- On the standard version of SIC, input and output are performed by + trasferring 1 byte at a time to or from the rightmost 8 bits of + register A. +- Each device has a 8bit code +- There are three I/O instructions +- Test Device (`TD`) tests whether the addressed device is ready; `CC` + is set to indicate the result ( < means the device is ready; = means + it's not ready) +- `RD` and `WD` are used to read and write data to the device. Reading + will store a byte into register A from the device and writing will + transfer a byte from register A to the device + + +## SIC/XE Machine Architecture + +### Memory + +- Max memory available on SIC/XE system is 1 megabyte ($2^{20}$ bytes). + +### Registers + +![Additional registers in SIC/XE](img/sic-xe-registers.png){width=65%} + + +### Data Formats + +- Additional 48 bit floating-point data type + +![Floating point format](img/floating-point-format.png){width=50%} + +- The fraction is interpreted as a value between 0 and 1; ie, the binary point + is assumed to be just before the high-order bit of the fraction. +- For normalized flaoting-point numbers, the high-order bit of the fraction + must be 1. +- The exponent is an unsigned binary number between 0 and 2047 +- If $f$ is the fraction and $e$ is the exponent, then the absolute value of + the number represeted is: $$f\times 2^{(e-1024)}$$ + + +### Instruction Formats + +- SIC/XE provides 4 formats of instructions (Refer Appendix A) + + +### Addressing Modes + +- Format 3 and 4 have flags to specify different addressing modes. +- Two new relative addrssing modes are available for use with *Format 3* + +![Relative addressing used with format 3](img/format-3-relative-addressing.png){width=70%} + +- In Format 3, for _base relative_ addressing _disp_ field is interpreted as a + 12-bit unsigned integer. For _program-counter relative_ addressing _disp_ + field is interpreted as a 12-bit **signed** integer, with negative values + represented in 2's comp. +- In Format 3, If _b_ and _p_ are set to zero then _disp_ is the target + address. In Format 4, _b_ and _p_ are normally zero and the target address is + taken from the address field. +- Any of theses modes can be combined with _indexed_ addressing; if bit _x_ is + set to 1, then (X) is added in target address calculation. +- **Immediate and Indirect addressing** \ + \- In Format 3 and 4, if bit _i_=1 and _n_=0 then the target address is + itself used as the operand value.(**i**mmediate addressing)\ + \- If bit _i_=0 and _n_=1, the word at the location given by the target address + is fetched; the _value_ contained in this word is then taken as the _address_ + of the operand value (i**n**direct addressing)\ + \- If _i_ and _n_ are both 0 or 1 then then the target of the address is taken + as the location of the operand (simple addressing)\ + \- **Indexing cannot be used with immediate or indirect addresssing**\ + \-SIC/XE assemblers will default to 1 for both _i_ and _n_ if the + instructions do not specify immediate or indirect addressing modes.\ + \-**Standard SIC assemblers will set the _i_ and _n_ bits both to zero + (this is because all the 8-bit binary codes for SIC instructions end with + 00); if bits _n_ and _i_ are both 0, then the bits _b_, _p_, and _e_ are + considered to be part of the address field of the instruction. This makes + Format 3 identical to the format used on the standard version SIC, providing + desired compatibility** + +![SIC/XE adressing modes example](img/sic-xe-addressing-modes-example.png) +