Memory Access
To access memory we can use these four registers: BX, SI, DI, BP. Combining these registers inside [ ] symbols, we can get different memory locations. These combinations are supported (addressing modes)
d8 - stays for 8 bit displacement.
d16 - stays for 16 bit displacement.
Displacement can be a immediate value or offset of a variable, or even both. It's up to compiler to calculate a single immediate value.
Displacement can be inside or outside of [ ] symbols, compiler generates the same machine code for both ways.
Displacement is a signed value, so it can be both positive or negative.
+ 25 = 1725. By default DS segment register is used for all modes except those with BP register, for these SS segment register is used. There is an easy way to remember all those possible combinations using this chart:
You can form all valid combinations by taking only one item from
each column or skipping the column by not taking anything from it. As you
see BX and BP never go together. SI and DI also don't go together. Here
is an example of a valid addressing mode: [BX+5].
In order to say the compiler about data type, these prefixes should be used:
BYTE PTR - for byte.
WORD PTR - for word (two bytes). For example:
BYTE PTR [BX] ; byte access.
or
WORD PTR [BX] ; word access.
MicroAsm supports shorter prefixes as well:
b. - for BYTE PTR
w. - for WORD PTR Sometimes compiler can calculate the data type automatically, but you may not and should not rely on that when one of the operands is an immediate value.
MOV instruction
· Copies the second operand (source) to the first operand (destination).
· The source operand can be an immediate value, general-purpose register or memory location.
· The destination register can be a general-purpose register, or memory location.
· Both operands must be the same size, which can be a byte or a word.
These types of operands are supported:
MOV REG, memory
MOV memory, REG
MOV REG, REG
MOV memory, immediate
MOV REG, immediate
REG: AX, BX, CX, DX, AH, AL, BL, BH, CH, CL, DH, DL, DI, SI, BP, SP.
memory: [BX], [BX+SI+7], variable, etc...
immediate: 5, -24, 3Fh, 10001101b, etc...
To access memory we can use these four registers: BX, SI, DI, BP. Combining these registers inside [ ] symbols, we can get different memory locations. These combinations are supported (addressing modes)
d8 - stays for 8 bit displacement.
d16 - stays for 16 bit displacement.
Displacement can be a immediate value or offset of a variable, or even both. It's up to compiler to calculate a single immediate value.
Displacement can be inside or outside of [ ] symbols, compiler generates the same machine code for both ways.
Displacement is a signed value, so it can be both positive or negative.
 |
| Memory access |
Generally the compiler takes care about difference between d8 and
d16, and generates the required machine code.
For example, let's assume that DS = 100, BX = 30, SI = 70.
The following addressing mode: [BX + SI] + 25
is calculated by processor to this physical address: 100 * 16 + 30 + 70+ 25 = 1725. By default DS segment register is used for all modes except those with BP register, for these SS segment register is used. There is an easy way to remember all those possible combinations using this chart:
You can form all valid combinations by taking only one item from
each column or skipping the column by not taking anything from it. As you
see BX and BP never go together. SI and DI also don't go together. Here
is an example of a valid addressing mode: [BX+5].
In order to say the compiler about data type, these prefixes should be used:
BYTE PTR - for byte.
WORD PTR - for word (two bytes). For example:
BYTE PTR [BX] ; byte access.
or
WORD PTR [BX] ; word access.
MicroAsm supports shorter prefixes as well:
b. - for BYTE PTR
w. - for WORD PTR Sometimes compiler can calculate the data type automatically, but you may not and should not rely on that when one of the operands is an immediate value.
MOV instruction
· Copies the second operand (source) to the first operand (destination).
· The source operand can be an immediate value, general-purpose register or memory location.
· The destination register can be a general-purpose register, or memory location.
· Both operands must be the same size, which can be a byte or a word.
These types of operands are supported:
MOV REG, memory
MOV memory, REG
MOV REG, REG
MOV memory, immediate
MOV REG, immediate
REG: AX, BX, CX, DX, AH, AL, BL, BH, CH, CL, DH, DL, DI, SI, BP, SP.
memory: [BX], [BX+SI+7], variable, etc...
immediate: 5, -24, 3Fh, 10001101b, etc...
For segment registers only these types of MOV are supported:
MOV SREG, memory
MOV memory, SREG
MOV REG, SREG
MOV SREG, REG
SREG: DS, ES, SS, and only as second operand: CS.
REG: AX, BX, CX, DX, AH, AL, BL, BH, CH, CL, DH, DL, DI, SI, BP, SP.
memory: [BX], [BX+SI+7], variable, etc...
SREG: DS, ES, SS, and only as second operand: CS.
REG: AX, BX, CX, DX, AH, AL, BL, BH, CH, CL, DH, DL, DI, SI, BP, SP.
memory: [BX], [BX+SI+7], variable, etc...
The MOV instruction cannot be used to set the value of the CS and IP registers
.
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