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Cx51 User's Guide
Cx51 Introduction
Compiling Programs
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Startup Code
Variable Initialization Code
XBANKING.A51
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Segment Naming Conventions
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Interfacing C to Assembler
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Passing in Memory
Passing in Registers
Function Return Values
Using the SRC Directive
Register Usage
Overlaying Segments
Small Model Example
Compact Model Example
Large Model Example
Interfacing C programs to PL/M-51
Data Storage Formats
Bit Variables
1-Byte Scalars
2-Byte Scalars
4-Byte Scalars
Generic and Far Pointers
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Absolute Memory Locations
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The _at_ Keyword
Debugging
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Library Reference
Appendix
Startup Code
Startup code is executed immediately upon reset of the target system. The Keil startup code performs (optionally) the following operations in order:
- Clears internal data memory
- Clears external data memory
- Clears paged external data memory
- Initializes the small model reentrant stack and pointer
- Initializes the large model reentrant stack and pointer
- Initializes the compact model reentrant stack and pointer
- Initializes the 8051 hardware stack pointer
- Transfers control to code that initializes global variables or to the main C function if there are no initialized global variables
Following is a list of the startup files that are available.
| Startup File | Description |
|---|---|
| STARTUP.A51 | Startup code for classic 8051 devices. |
| START_AD.A51 | Startup code for Analog Devices MicroConverter devices. |
| START390.A51 | Startup code for Dallas DS80C390/400/41x/5240/5250 contiguous mode devices. |
| START4XX.A51 | Startup code for Dallas DS89C420/430/440/450 devices. |
| STARTLPC.A51 | Startup code for NXP LPC700 devices series. |
| START900.A51 | Startup code for NXP LPC90x - LPC93x devices series. |
| START950.A51 | Startup code for NXP LPC95x - LPC99x devices series. |
| START_MX.A51 | Startup code for NXP 80C51MX devices. |
| START751.A51 | Startup code for NXP 80C75x devices. |
| START_XC.A51 | Startup code for Infineon XC800 devices. |
| STARTUP32.A51 | Startup code for ST uPSD32xx devices. |
| STARTUP34.A51 | Startup code for ST uPSD33xx devices. |
| STARTUP34.A51 | Startup code for ST uPSD34xx devices. |
Copy the appropriate startup file from the \KEIL\C51\LIB\ folder into your project folder and make any changes to the copy.
Each startup file provides assembly constants you may change to control the operations performed at startup.
| Name | Description |
|---|---|
| IDATALEN | Specifies the number of bytes of idata to clear to 0. The default is 80h because most 8051 derivatives contain at least 128 bytes of internal data memory. Use a value of 100h for the 8052 and other derivatives that have 256 bytes of internal data memory. |
| XDATASTART | Specifies the initial xdata address to clear to 0. |
| XDATALEN | Indicates the number of bytes of xdata to clear to 0. The default is 0. |
| PDATASTART | Specifies the initial pdata address to clear to 0. |
| PDATALEN | Specifies the number of bytes of pdata to clear to 0. The default is 0. |
| IBPSTACK | Specifies whether or not the small model reentrant stack pointer (?C_IBP) should be initialized. A value of 1 causes this pointer to be initialized. A value of 0 prevents initialization of this pointer. The default is 0. |
| IBPSTACKTOP | Specifies the top of the small model reentrant
stack. The default is 0xFF in idata memory. The Cx51 Compiler does not check to see if the stack area available satisfies the requirements of the application. It is your responsibility to perform such a test. |
| XBPSTACK | Specifies whether or not the large model reentrant stack pointer (?C_XBP) should be initialized. A value of 1 causes this pointer to be initialized. A value of 0 prevents initialization of this pointer. The default is 0. |
| XBPSTACKTOP | Specifies the top of the large model reentrant
stack. The default is 0xFFFF in xdata memory. The Cx51 Compiler does not check to see if the stack area available satisfies the requirements of the application. It is your responsibility to perform such a test. |
| PBPSTACK | Specifies whether the compact model reentrant stack pointer (?C_PBP) should be initialized. A value of 1 causes this pointer to be initialized. A value of 0 prevents initialization of this pointer. The default is 0. |
| PBPSTACKTOP | Specifies the top of the compact model reentrant
stack. The default is 0xFF in pdata memory. The Cx51 Compiler does not check to see if the stack area available satisfies the requirements of the application. It is your responsibility to perform such a test. |
| PPAGEENABLE | Enables (a value of 1) or disables (a value of 0) Port 2 initialization for pdata memory access. The default is 0. pdata addressing uses Port 2 for the upper address (or page) byte. |
| PPAGE |
Specifies the value to write to Port 2 of the 8051 for pdata memory access. This value represents the xdata memory page to use for pdata. This is the upper 8 bits of the absolute address range to use for pdata. For example, if the pdata area begins at address 1000h (page 10h) in xdata memory, PPAGEENABLE should be set to 1, and PPAGE should be set to 10h. You must specify the starting pdata address to use to the BL51 Linker using the PDATA directive. For example: BL51 input modules PDATA (1050H) Neither the BL51 Linker nor the Cx51 Compiler checks to see if the PDATA directive and the PPAGE startup constant are correctly specified. You must ensure that these parameters contain suitable values. |
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