system-software-beck/01-background.md
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---
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...
## 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
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### 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.
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### Addressing Modes
- two addressing modes available indicated by the $x$ bit
- (X) means the contents of the register X
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### 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
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### Data Formats
- Additional 48 bit floating-point data type
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- 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*
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- 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**
